Pin matching for local variables and constants is already supported,
and it is fairly simple to add support for these variable types.
Note that pin matching for method calls is still not supported
without wrapping in parentheses (pin expressions). I think that's
for the best as method calls are far more complex (arguments/blocks).
Implements [Feature #17724]
Add separate doc/syntax/pattern_matching.rdoc, add
link to control_expressions.rdoc.
The documentation is "reverse-engineered" from Ruby 2.7
behavior and early preview presentations, and corrected
by pattern-matching feature author @k-tsj.
