crypto/x509: clarify docs for SystemCertPool

The sentence in the docs for SystemCertPool that states that mutations
to a returned pool do not affect any other pool is ambiguous as to who
the any other pools are, because pools can be created in multiple ways
that have nothing to do with the system certificate pool. Also the use
of the word 'the' instead of 'a' early in the sentence implies there is
only one shared pool ever returned.

Fixes #27385

Change-Id: I43adbfca26fdd66c4adbf06eb85361139a1dea93
GitHub-Last-Rev: 2f1ba09fa403d31d2d543dca15727c6c2f896ec7
GitHub-Pull-Request: golang/go#27388
Reviewed-on: https://go-review.googlesource.com/132378


Reviewed-by: Filippo Valsorda <filippo@golang.org>