manual: clarify buffer behavior in getline [BZ #5666]

If the user has requested automatic buffer creation, getline may create
it and not free things when an error occurs.  That means the user is
always responsible for calling free() regardless of the return value.

The current documentation does not explicitly cover this which leaves it
slightly ambiguous to the reader.  So clarify things.

URL: https://sourceware.org/bugzilla/show_bug.cgi?id=5666