doc: Clarify when extracted comments get extracted.
Fixes the fix from <https://savannah.gnu.org/bugs/index.php?42376>.
Prompted by <https://savannah.gnu.org/bugs/index.php?64136>.
* gettext-tools/doc/xgettext.texi: Clarify that it's not blank lines, but lines
with tokens, that prevent the extraction of a preceding comment block.
