Change a method name, for clarity.

[hjoliver]

A tiny non-functional code-tidy change for clarity.

• change release_held_active_task(itask) to release_held_task(itask)
• and add to the method's docstring

This method is not just called for active (n=0) tasks.

The task proxy argument can also be a transient (a temporary task proxy not intended to be added to the task pool, but used for other purposes) representing a future (n>0, non-active) task.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.