Documentation of help-echo events

[Eli Zaretskii]

I see that commands.texi doesn't include help-echo events in the list
documented in the "Input Events" section.  All it needs to do is to
refer to the node "Special Properties" that describes the help-echo
property, which has the same form and is treated in the same way as
help-echo events found in the keyboard buffer.

Does this omission have a purpose, and if so, what is it?  Or should I
add the missing xref?