bug#13326: document how to include more types of error messages in view-echo-area-messages

[jidanni]

We see

   C-h e runs the command view-echo-area-messages, which is an
   interactive compiled Lisp function in `help.el'.

   It is bound to C-h e, <f1> e, <help> e.

   (view-echo-area-messages)

   View the log of recent echo-area messages: the `*Messages*' buffer.
   The number of messages retained in that buffer
   is specified by the variable `message-log-max'.

However underfed key messages don't end in there and indeed have no way
to be captured. So perhaps the above description should mention how to
capture them,