Many gtest users aren't aware that a polymorphic matcher is not actually a `testing::Matcher<>` and thus doesn't know how to describe itself. Clarify this.
Also adds a recipe on how to define a composite matcher. In particular, explain how the composite matcher can describe itself in terms of the description(s) of its sub-matcher(s).
PiperOrigin-RevId: 735840759
Change-Id: I26cff6179349aa739fc7fdd528a2f5308d18c189