Update docstrings in the fcntl module

[serhiy-storchaka]

• They still refer to string arguments as if it is Python 2.
• They still refer to the limitation of 1024 bytes.
• They sporadically use backticks to denote arguments, even if it is not Markdown.
• Some lines are longer than 80 columns in the pydoc output.

Linked PRs

• gh-140615: Update docstrings in the fcntl module #140619