We always supported underscores in tags since \w in pythons
interpretation of regular expressions does include the underscore.
While this might have happened by accident, there is no reason to
change the implementation. The motivation for the restriction
was to avoid accidents with wrong (manual) shell escaping; the
underscore, however, has no special meaning to the shell. So
just make the documentation say what we implemented.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>