Fix Graphql/Descriptions EE mutation offenses

What does this MR do?

!46721 (merged) introduced a new rubocop that enforced periods at the end of GraphQL description strings. Existing offenses were added to .rubocop_manual_todo.yml.

This change fixes offenses only for files in ee/app/graphql/mutations and ee/app/graphql/ee/mutations, and removes the exclusion of them from .rubocop_manual_todo.yml.

The changes were generated using autocorrect:

    bundle exec rubocop ee/app/graphql \
      --auto-correct \
      --only Graphql/Descriptions

• Issue #292426 (closed)

Reviewing tips

1. Single-file reviewing might be useful (on the Changes tab, click the cog icon, then check Show one file at a time).
2. The total number of changes lines in this MR is very high, but this is mainly due to the changes under docs/ which are auto-generated (and verified during CI).

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Related to #292426 (closed)