Documentation: bcachefs: casefolding: Fix dentry/dcache considerations section

Documentation: bcachefs: casefolding: Fix dentry/dcache considerations section

Sphinx reports htmldocs warnings on dentry/dcache section:

Documentation/filesystems/bcachefs/casefolding.rst:75: WARNING: Title underline too short.

dentry/dcache considerations
--------- [docutils]
Documentation/filesystems/bcachefs/casefolding.rst:84: WARNING: Definition list ends without a blank line; unexpected unindent. [docutils]

Fix the section by:

* Extending the section underline to match the section title length;
* Separating problem list from surrounding paragraphs.

Fixes: bc5cc09246c5 ("bcachefs: bcachefs_metadata_version_casefolding")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250221161911.2d16138b@canb.auug.org.au/
Closes: https://lore.kernel.org/linux-next/20250221162135.79be0147@canb.auug.org.au/
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>