encoding/json: fix typos, grammar, and stale comment references

Fixes covering article errors (a/an before vowel-sound words), subject-verb
agreement, missing or duplicated words, inverted inline-comment logic, stale
identifier references in comments after renames, out-of-date doc strings, and a
stray identifier typo.

Change-Id: I46fbe5b990bd8902f4b0722776a1c587a7b55141
Reviewed-on: https://go-review.googlesource.com/c/go/+/775480
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>