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>

Author: pks-t

Documentation/glossary-content.txt

@@ -498,8 +498,8 @@ exclude;;
 
 [[def_pseudoref]]pseudoref::
 	A ref that has different semantics than normal refs. These refs can be
-	accessed via normal Git commands but may not behave the same as a
-	normal ref in some cases.
+	read via normal Git commands, but cannot be written to by commands like
+	linkgit:git-update-ref[1].
 +
 The following pseudorefs are known to Git: