[rr-]

The short description is now the whole first paragraph, in every style, instead of only its first line. Users with summaries wrapped over two lines get the whole sentence back, and the long description no longer starts halfway through it.

Before:

>>> d = parse("Return the :class:`Foo` for\n:meth:`bar` calls.\n\nMore.")
>>> d.short_description
'Return the :class:`Foo` for'
>>> d.long_description
':meth:`bar` calls.\n\nMore.'

After:

>>> d.short_description
'Return the :class:`Foo` for\n:meth:`bar` calls.'
>>> d.long_description
'More.'

The paragraph ends at the first blank line. It also ends at an indented line, since that starts a block, and at a section title or field, which end it already. Compose writes a multi-line short description back as it was read, so it round-trips. A line holding only whitespace before more text now counts as the blank line after the short description, so that compose writes it out and the docstring reads back the same.

This is a breaking change: code that expects short_description to be one line will now see newlines in it.

Parity against main: 11743 of 29158 docstrings differ (430 case, 1646 stdlib, 9667 generated). I checked samples from each group, and every diff comes from a summary that spans several lines. The round-trip invariant passes with no new exclusions.

Bench against main: 1.03x at most (parse epydoc). split_description scans the paragraph with one regex.