Rewrite generated docstrings in indexer/watcher.py

[mercael91]

Rewrites the generated docstrings listed in #135 with plain descriptions of what each method does, following the #131 register.

Docstrings only in palinode/indexer/watcher.py — no behaviour changes, no new functions, no deletions. Human-written docstrings (e.g. on_moved, shutdown_handlers) are left untouched.

[Paul-Kyle]

Merging — this is exactly the shape the issue asked for, and I checked it against the list rather
than taking the description's word: every line from the table is covered, the human-written
docstrings are untouched, and nothing outside a docstring moved.

"Return True if path is a Markdown file outside ignored directories" is a good example of what the
whole file needed — it tells a reader the two things that actually decide the answer.

You spotted the earlier one yourself and turned it around inside a quarter of an hour, which is
worth more to me than a first attempt that happened to be clean. Thanks for coming back to it.