Improve Vale's ElementDescriptors rule to catch previous false negatives

What does this MR do?

Closes #408574 (closed). Related to !106846 (comment 1208574203).

This MR improves Vale's ElementDescriptors rule to:

• report the start-of-occurrence position
• improve the efficiency by approximately 500%
• catch previous false negatives (where button is on the next line, or the text inside ** is on the next line)

  **Status checks** sub-section, select the **Add status
  check** button
  **Status checks** sub-section, select the **Add status check**
  button

  • one such example is import_export.md:121. There are 5 such cases in the current documentation

Command

• vale doc

Sample lint

Before: \*\*.+?\*\* button, 6 matches, 3991 steps

doc/user/project/merge_requests/status_checks.md

88:12  - **Status checks** sub-section, select the **Add status check** button
93:39  - <same>
97:12  - **Status checks** sub-section, select the **Edit** button
103:51 - <same>
140:12 - **Status checks** sub-section, select the **Remove...** button
147:1  - <same>

After: \*\*[^*]+\*\*\s+button, 6 matches, 685 steps

doc/user/project/merge_requests/status_checks.md

88:54  - **Add status check** button
93:39  - **Add status check** button
97:54  - **Edit** button
103:51 - **Update status check** button
140:54 - **Remove...** button
147:1  - **Remove status check** button

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.