Clarify documentation of GzHeader::mtime

[dtolnay]

Returns the `mtime` field of this gzip stream's header, if present.

The mtime field is always required to be present. But it is 32 bits so because of the 2038 problem its use is discouraged. Most libraries set it to 0 and never look at it.

Document that the usage of mtime is discouraged and that 0 is the expected value for modern libraries.