docs: Add explanation of when to use 'warn' severity (#16882)

* docs: Add explanation of when to use 'warn' severity

Fixes #16696
Closes #16753

* Update docs/src/use/configure/rules.md

Co-authored-by: Nitin Kumar <[email protected]>

---------

Co-authored-by: Nitin Kumar <[email protected]>

Author: nzakas

docs/src/use/configure/rules.md

@@ -20,6 +20,10 @@ To change a rule's severity, set the rule ID equal to one of these values:
 * `"warn"` or `1` - turn the rule on as a warning (doesn't affect exit code)
 * `"error"` or `2` - turn the rule on as an error (exit code is 1 when triggered)
 
+Rules are typically set to `"error"` to enforce compliance with the rule during continuous integration testing, pre-commit checks, and pull request merging because doing so causes ESLint to exit with a non-zero exit code.
+
+If you don't want to enforce compliance with a rule but would still like ESLint to report the rule's violations, set the severity to `"warn"`. This is typically used when introducing a new rule that will eventually be set to `"error"`, when a rule is flagging something other than a potential buildtime or runtime error (such as an unused variable), or when a rule cannot determine with certainty that a problem has been found (when a rule might have false positives and need manual review).
+
 ### Using configuration comments
 
 To configure rules inside of a file using configuration comments, use a comment in the following format: