docs: Fix Sphinx 3.x warnings, and enable warnings-as-errors on build.

This enables warnings as errors and fixes all current errors, namely:

- reference to terms in the glossary must now be explicit (:term:)
- method overloads must not be declared as a separate method or must
  use :noindex:
- 2 cases where `` should have been used instead of `

docs/Makefile

@@ -3,7 +3,7 @@
 
 # You can set these variables from the command line.
 PYTHON        = python3
-SPHINXOPTS    =
+SPHINXOPTS    = -W --keep-going
 SPHINXBUILD   = sphinx-build
 PAPER         =
 BUILDDIR      = build/$(MICROPY_PORT)