docs: troubleshooting with "autorelease: pending" label (#1982)

Co-authored-by: Jeff Ching <chingor@google.com>

README.md

@@ -139,6 +139,8 @@ recommend using squash-merge instead](#linear-git-commit-history-use-squash-merg
 
 ## Release Please bot does not create a release PR. Why?
 
+### Releasable Units
+
 Release Please creates a release pull request after it notices the default branch
 contains "releasable units" since the last release.
 A releasable unit is a commit to the branch with one of the following
@@ -148,12 +150,24 @@ prefixes: "feat", "fix", and "deps".
 Some languages have their specific releasable unit configuration. For example,
 "docs" is a prefix for releasable units in Java and Python.
 
+### Re-run Release Please
+
 If you think Release Please missed creating a release PR after a pull request
 with a releasable unit has been merged, please re-run `release-please`. If you are using
 the GitHub application, add `release-please:force-run` label to the merged pull request. If
 you are using the action, look for the failed invocation and retry the workflow run.
 Release Please will process the pull request immediately to find releasable units.
 
+### Existing Pull Requests with "autorelease: pending" label
+
+For the GitHub application users, Release Please will not create a new pull request
+if there's an existing pull request labeled as `autorelease: pending`.
+To confirm this case, search for a pull request with the label.
+(It's very likely it's the latest release pull request.)
+If you find a release pull request with the label and it is not going to be released
+(or already released), then remove the `autorelease: pending` label and re-run Release
+Please.
+
 ## Strategy (Language) types supported
 
 Release Please automates releases for the following flavors of repositories: