#1623 Document undefined bindings

[Anonymous]

The `bind` man page has always said, "Some of the
substitutions are only valid for certain types of
events; if they are used for other types of events the
value substituted is undefined." Donal has been
tightening this up lately (8.4?), so that now invalid
substitutions are guaranteed to yield {???} (true?). I
ask that we upgrade the documentation to reflect this.

Donal believes Joe English is likely to be in a good
position to review the idea.

[Donal K. Fellows]

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FWIW, in the longer term I'd love to parse events at [bind]
time and throw an error if they have invalid substitutions.
That's a Tk9 thing at the earliest though.