chore(dev): Detail the format of DEPRECATIONS.md file

The format of this file was originally described in #14983

Signed-off-by: Jesse Szwedko <jesse.szwedko@datadoghq.com>

docs/DEPRECATIONS.md

@@ -1,5 +1,13 @@
 See [DEPRECATION.md](docs/DEPRECATION.md#process) for the process for updating this file.
 
+The format for each entry should be: `<version> <identifier> <description>`.
+
+- `<version>` should be the version of Vector in which to take the action (deprecate, migrate, or
+  remove)
+- `<identifier>` should be a unique identifier that can also be used in the code to easily find the
+  places to modify
+- `<description>` should be a longer form description of the change to be made
+
 ## To be deprecated
 
 ## To be migrated