The inspect format was intentionally changed as an outcome of
[Bug #20433] [ruby-core:118668], but some documentation update
was missing, as [Bug #20962] pointed out. Update some output
examples that clearly use Hash#inspect.
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.
