doc: clarify tail -n/-c +NUM operation

doc: clarify tail -n/-c +NUM operation

tail -n/-c +NUM, is different from tail -n/-c NUM,
and head -n/-c NUM, and head -n/c -NUM, in that it
specifies a 1 based index rather than a count to skip/include.
So clarify this in tail --help and tail info manual.
Note we also mention this gotcha at:
https://www.pixelbeat.org/docs/coreutils-gotchas.html#tail

* doc/coreutils.texi (tail invocation): Give examples for -c/-n +NUM,
to make it clear one has to specify a number 1 larger than
might be expected.
* src/tail.c (usage): State the skip at start edge case more clearly
in the -n description. -c is not often used with tail so we leave
full explanation of that to the info manual.  Also split the string
to simplify translation.