pp_ctl.c: revamp S_docatch() and docs

Document fully what S_docatch() does, and remove the one macro which
calls it, replacing all the call-sites with a direct call to docatch().

docatch() is a core part of perl's internal exception handling, and as
such is conceptually complex. It was made even more complex by
v5.27.0-75-gd7e3f70f30, which causes functions like pp_entertry() to
recursively call themselves sometimes. But this detail was hidden by
the RUN_PP_CATCHABLY() macro, which included a hidden 'return'.
By unwrapping this trivial macro, it makes the flow of control much
easier to understand.

I've also added an extra assert(!CATCH_GET), to emphasise that this
setting is reset following the JMPENV_PUSH() call.

Apart from that, there should be no functional differences.

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