Documentation/glossary: clarify limitations of pseudorefs

Documentation/glossary: clarify limitations of pseudorefs

Clarify limitations that pseudorefs have:

  - They can be read via git-rev-parse(1) and similar tools.

  - They are not surfaced when iterating through refs, like when using
    git-for-each-ref(1). They are not refs, so iterating through refs
    should not surface them.

  - They cannot be written via git-update-ref(1) and related commands.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>