time: document that valid layouts are not valid Parse values

time: document that valid layouts are not valid Parse values

For #9346 #22135 explicitly state under layout constants
that they are not valid time values for Parse. Also add
examples of parsing valid RFC3339 values and the layout
to the example for time.Parse.

Fix capitalisation of time.Parse and Time.Format.

For #20869 include RFC3339 in the list of layouts that do
not accept all the time formats allowed by RFCs (lowercase z).
This does not fully address #20869.

Fixes #9346
Fixes #22135

Change-Id: Ia4c13e5745de583db5ef7d5b1688d7768bc42c1b
Reviewed-on: https://go-review.googlesource.com/74231
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>