Address Copilot review on find_or_create_pull_request

- Reword the action's details: base is required (drop 'optionally targeting a
  specific base') to match the required `base` ConfigItem.
- find_pull_request YARD: mark `base` as `[String?]` since it accepts nil.

Author: oguzkocer

lib/fastlane/plugin/wpmreleasetoolkit/actions/common/find_or_create_pull_request_action.rb

@@ -42,7 +42,7 @@ def self.description
 
       def self.details
         <<~DETAILS
-          Looks for an open Pull Request whose head is the given branch (optionally targeting a specific base),
+          Looks for an open Pull Request whose head is the given branch and which targets the given base,
           and returns its URL if found. Otherwise, creates a new Pull Request and returns its URL.
 
           This is useful for "rolling" automations (e.g. a daily translations or dependency-update job) that

lib/fastlane/plugin/wpmreleasetoolkit/helper/github_helper.rb

@@ -309,7 +309,7 @@ def comment_on_pr(project_slug:, pr_number:, body:, reuse_identifier: SecureRand
       # @param [String] repository The repository name, including the organization (e.g. `wordpress-mobile/wordpress-ios`)
       # @param [String] head The head branch to look for. May be given as `branch` or as the fully-qualified `owner:branch`;
       #        when unqualified, it is automatically prefixed with the repository's owner.
-      # @param [String] base The base branch the PR should target. If nil, PRs targeting any base are considered.
+      # @param [String?] base The base branch the PR should target. If nil, PRs targeting any base are considered.
       # @param [String] state The PR state to match (`open`, `closed`, or `all`). Defaults to `open`.
       # @return [Sawyer::Resource, nil] The first matching Pull Request, or nil if none matches.
       #