doc: add `seq inf` and `sleep inf` examples to texinfo

doc: add `seq inf` and `sleep inf` examples to texinfo

* doc/coreutils.texi (seq invocation): Mention "inf" is supported,
and describe that it's handled specially to generate infinite
whole integer sequences.  Also mention that such infinite generation
is supported for integer steps up to 200.
(sleep invocation): Give `sleep inf` as an example to sleep forever.
* src/seq.c: Add a comment on SEQ_FAST_STEP_LIMIT, to say it's
reflected in the texinfo description.