doc/go1.17: document text/template/parse.SkipFuncCheck

Documents the newly added mode that skips type checking
functions as per CL 301493.

Fixes #46025
For #34652
For #44513
For #38627

Change-Id: I56c4f65924702a931944796e39f43cfeb66abc8a
Reviewed-on: https://go-review.googlesource.com/c/go/+/311569


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Michael Knyszek <mknyszek@google.com>