📝 create search config details document

Signed-off-by: vankichi <kyukawa315@gmail.com>

docs/user-guides/operations.md

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 This page introduces best practices for operating a Vald cluster.
 
-
-
 ## Deployment
 
 ### Kubernetes cluster

docs/user-guides/search-config.md

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+# Search Config
+
+## Vald Search Service
+
+<!-- Describe vald search service: search, searchbyid, linearsearch, lineasearchbyid ... -->
+
+Vald provides the two types of search service.
+
+1. Search
+
+   - `Search` is just the `ANN(Approximate Nearest Neighbor)` search with query vector.
+     It is the fast search even the vector consists large dimension.
+     The search duration is fast, but less accurate than `LinearSearch`.
+     Search algorithm depends on each core algorithm.
+
+1. SearchById
+
+   - `SearchById` is just the `ANN(Approximate Nearest Neighbor` search with vector's id.
+     The id should be already indexed before search process.
+     The sarch algorithm is the same as `Search`.
+
+1. LinearSearch
+
+   - `LinearSearch` is the basic search algorithm with query vector.
+     It searches all indexed vectors and calculates the distance between the query.
+     Its accuracy is exact, but the search time requires more than `Search` (ANN search) and increases the amount of indexed vector.
+
+1. LinearSearchById
+   - `LinearSearchById` is the basic search algorithm with vector's id.
+     The id should be already indexed before search process.
+     The sarch algorithm is the same as `LinearSearch`.
+
+<div class="notice">
+Linear Search service is available from Vald v1.4 or later.
+</div>
+
+For more details, please refer to [Search API document](../api/search.md)
+
+## Configuration
+
+<!-- Describe Search parameters: ../../apis/proto/v1/payload -->
+<!-- TODO: divide search configuration for each agent algorithm -->
+
+Here is the current search config parameters.
+
+```bash
+// Represent search configuration.
+message Config {
+  // Unique request ID.
+  string request_id = 1;
+  // Maximum number of result to be returned.
+  uint32 num = 2 [ (validate.rules).uint32.gte = 1 ];
+  // Search radius.
+  float radius = 3;
+  // Search coefficient.
+  float epsilon = 4;
+  // Search timeout in nanoseconds.
+  int64 timeout = 5;
+  // Ingress filter configurations.
+  Filter.Config ingress_filters = 6;
+  // Egress filter configurations.
+  Filter.Config egress_filters = 7;
+  // Minimum number of result to be returned.
+  uint32 min_num = 8 [ (validate.rules).uint32.gte = 0 ];
+}
+```
+
+### request_id
+
+`request_id` is unique request ID.
+It is **NOT** indexed vector's id.
+It can be used for e.g., error handling process.
+
+### num
+
+`num` is the maximum number of search result which you'd like to get.
+`num` should be positive integer.
+
+### radius
+
+`radius`, the specific parameter for NGT, specifies the search range centered on the query vector in terms of the radius of a circle.
+The number of search target vectors increases along with the radius is large.
+There is the trade-off between accuracy and search speed.
+In many cases, it is hard to set it due to depending on dataset.
+
+The default value is infinite circle.
+When setting negative number as `radius`, `NGT` applies the radius as infinite circle.
+
+<div class="notice">
+NGT will self-update radius during search process.
+</div>
+
+### epsilon
+
+`epsilon`, the specific parameter for NGT, specifies the magnification coefficient (epsilon) of the search range.
+NGT will use `(1+epsilon)*radius` as the search range.
+The number of search target vectors increases along with the epsilon is large.
+
+The default value (recommend value) is `0.1`.
+While it is desirable to adjust this value within the range of 0 - 0.3, a negative value (over than `-1`) may also be specified.
+
+### ingress_filters
+
+`ingress_filters` is required when using ingress filter component.
+It requires ingress filter component's hostname and port.
+
+### egress_filters
+
+`egress_filters` is required when using egress filter component.
+It requires egress filter component's hostname and port.
+
+### min_num
+
+`min_num` is the minimum number of search result which you'd like to get at least.
+It helps you to avoid getting the timeout error when the search process requires more time.
+`min_num` should be positive integer and smaller than `num`.