arch: Wrap a docstring in isa_parser.py.

arch: Wrap a docstring in isa_parser.py.

This brings the ISA parser in line with the style guide. Note that the
docstring needs to be a single string literal for python to consider it
a docstring, and the parser itself needs each line of the docstring to
be a rule in its CFG. We can accomplish both by taking advantage of the
fact that two directly adjacent quoted strings are treated as a single
string literal by python, and by escaping the newline so that they're
actually considered adjacent.

Change-Id: I7f4d252998877808425aafb0159600ba4c3bf9ad
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35276
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>