docs: document the YR_RE_SCAN_LIMIT regular expression scan limit

[munzzyy]

Fixes #2194.

The regexp engines cap how much data they scan at YR_RE_SCAN_LIMIT (4096 bytes, defined in libyara/include/yara/limits.h), so a match that needs more than that simply isn't reported. @plusvic pointed at the constant in the issue thread, but the limit isn't mentioned anywhere in the docs, which was the reporter's actual complaint: rules with wide wildcards fail silently and there's nothing in the manual explaining why.

This adds a warning box to the "Regular expressions" section of writingrules.rst stating the default limit, the silent-miss behavior, and that it can be raised at compile time.

Checked against HEAD (fa372eb) before writing the note:

• built yara and ran /foo.*bar/ over a file with foo and bar 4000 bytes apart (matches) and one with them 5000 bytes apart (no match, no error, exit 0)
• rebuilt with CPPFLAGS='-DYR_RE_SCAN_LIMIT=8192': the 5000-byte gap now matches and a 9000-byte one doesn't, so the compile-time override wording is accurate
• sphinx-build runs with no new warnings and the box renders at the end of the regexp section