Add more content related to process types (#737)

Signed-off-by: Hanan Younes <hanan.algazaley@gmail.com>

content/docs/for-buildpack-authors/how-to/write-buildpacks/specify-launch-processes.md

@@ -7,6 +7,39 @@ One of the benefits of buildpacks is that they are multi-process - an image can
 
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+A `process type` is a named process definition, contributed by a buildpack at build-time and executed by the launcher at run-time.
+Buildpacks declare process types during the build phase by writing entries into `<layers>/launch.toml`.
+
+## Key Points
+
+For each process, the buildpack:
+
+* MUST specify a `type`, an identifier for the process, which:
+  * MUST NOT be identical to other process types provided by the same buildpack.
+  * MUST only contain numbers, letters, and the characters `.`, `_`, and `-`.
+* MUST specify a `command` list such that:
+  * The first element of `command` is a path to an executable or the file name of an executable in `$PATH`.
+  * Any remaining elements of `command` are arguments that are always passed directly to the executable [^command-args].
+* MAY specify an `args` list to be passed directly to the specified executable, after arguments specified in `command`.
+  * The `args` list is a default list of arguments that may be overridden by the user [^command-args].
+* MAY specify a `default` boolean that indicates that the process type should be selected as the [buildpack-provided default](https://github.com/buildpacks/spec/blob/main/platform.md#outputs-4) during the export phase.
+* MAY specify a `working-dir` for the process. The `working-dir` defaults to the application directory if not specified.
+
+## Implementation Steps  
+
+Processes are added to the `launch.toml` file in the `<layers>/<layer>` directory as follows:
+
+```toml
+[[processes]]
+type = "<process type>"
+command = ["<command>"]
+args = ["<arguments>"]
+default = false
+working-dir = "<working directory>"
+```
+
+### Examples
+
 Let's see how this works. We will specify a process type that allows a debugger to attach to our application.
 
 To enable running the debug process, we'll need to have our buildpack define a "process type" for the worker.

content/docs/for-buildpack-authors/how-to/write-buildpacks/use-exec.d.md

@@ -13,7 +13,7 @@ The [buildpacks `exec.d` interface](https://github.com/buildpacks/spec/blob/main
 
 2. Script Behavior:
     * **Inputs**
-        *A third open file descriptor. File descriptors are integers used by a process to uniquely identify opened files; pre-opened file descriptors are usually 0 for stdin, 1 for stdout and 2 for stderr. The third open file descriptor is inherited from the calling process.  
+        * A third open file descriptor. File descriptors are integers used by a process to uniquely identify opened files; pre-opened file descriptors are usually `0` for `stdin`, `1` for `stdout` and `2` for `stderr`. The third open file descriptor is inherited from the calling process.  
     * **Outputs**
         * Valid TOML describing environment variables in the form of key=value pairs. These variables are added to the application's runtime environment. The content should be written to file descriptor 3 (see examples for how to do this).
         * Exit Code: The scripts should exit with a status code of `0` to indicate success. A non-zero exit code will indicate an error and prevent the application from launching.