docs: add note to wall(1) about sessions which will not get message

Sessions that has in utmp data ut_line begining with ':' are skipped.
This is done to avoid write errors to devices such as '/dev/:0'.  For
example wdm is reported to to generate such utmp line.

Perhaps in future this code should be revalidated.  If an utmp data
contains broken device lines it might be best inform user with a error
message, rather than skipping silently suspicious device entries.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

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