Sv: Better documentation for non-binding clauses of if-let and friends

[arthur miller <arthur.miller@live.com>]

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>I just recently wrote a simple RDP that has a couple of functions of this
>form:
>
>```
>(if-let* (((parsebib--char "@"))
>          ((parsebib--keyword '("string")))
>          (open (parsebib--char "{("))
>          (definition (parsebib--assignment))
>          ((parsebib--char (alist-get open '((?\{ . "}") (?\( . ")"))))))
>    definition
>  (signal 'parsebib-error (list (format "Malformed @String definition at position %d,%d"
>                                        (line-number-at-pos) (current-column)))))
>```
>
>This reads a complete BibTeX @String definition. The clauses in the
>`if-let*` specify what text should be read, and I can capture the result of
>those reads, but only for the parts that I need to capture.
>
>I think once you know how `if-let*` works, this is a fairly concise and
>quite readable way to code this rule. Not sure what the equivalent without
>`if-let*` would look like, TBH. (Short of creating a DSL, which would
>admittedly be nicer...)

In my opinion, if-let and when-let are no brainer. they are actualy useful as a
synctatic-sugar and help keep variables in the scope of ift- and when-let. Those
bindings are usable as ordinary let-bindings, since they are established only once.

Problem is when one introduce repetition: while-let re-establishes the bindings a new
for each repetition, which is a bit unusual I believe.

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