doc: update tail's documentation to allow for new -F semantics

doc: update tail's documentation to allow for new -F semantics

* src/tail.c (usage): Reword tail -F description, so that it no
longer mentions details specific to the non-inotify implementation.
Also, join diagnostic strings (while staying under the 509-byte limit)
to ease formatting of translations.  The latter was prompted by
a report from Stéphane Raimbault.
* doc/coreutils.texi (tail invocation): Update description here, too.