Update documentation for `DateTimeParse`'s format strings.
Update documentation for `DateTimeParse`'s format strings.

Many of the examples were misleading or inconsistent with what was actually supported.

RELNOTES: n/a
PiperOrigin-RevId: 448358083
Change-Id: I965a1055283357facbadab891ca344d395f81e28

google/closure-library,google/closure-library,google/closure-library,google/closure-library,google/closure-library