Tidy up a small bit of POD

Fix two typos and link the DatabaseType option.
Don't check case sensitivity of Limit when the value isn't quoted

An unquoted value means raw SQL which won't be made case-insensitive by
DBIx::SearchBuilder anyway (refer to _GenericRestriction).

This ensures that the 4.1.13 backcompat file doesn't erroneously produce
case-sensitivity warnings.
Define a filename and line number when eval-ing backcompat files

This greatly improves the ability to debug backcompat files given a
stacktrace.  It turns "at eval (42) line 5" into a real filename and
correct line number.
Merge branch '4.2/less-sensitive-case-sensitive-check'