perldiag: Update descr for ‘Switch condition not recognized’

perldiag: Update descr for ‘Switch condition not recognized’

Originally, the message ‘Switch (?(number%c not recognized’ only
applied to /(?(1junk))/.  perl-5.6.0-657-gb45f050 changed it to read
‘Switch condition not recognized’.

perl-5.8.0-8771-g0a4db38 added (?(<...>)), (?('...')), (?(DEFINE)) and
(?(R...)).  All of these can trigger that messages if there is junk
before the first closing parenthesis (e.g., /(?(<name>junk))/, causing
a mismatch between the cases where the error occurs and the descrip-
tion in perldiag:

    (F) If the argument to the (?(...)if-clause|else-clause) con-
    struct is a number, it can be only a number.  The <-- HERE shows
    whereabouts in the regular expression the problem was discovered.
    See perlre.

(which itself was already confusingly worded).

Whether these should be changed to use the more standard ‘Unknown
switch condition’ message I don’t want to deal with right now.

In any case, the description is only sometimes relevant, so this com-
mit just copies the description from ‘Unknown switch condition’.