Ignore some commands when generating devtest.md.

Running devtest.sh will require some commands that don't make sense
in the devtest.md docs, such as toggling set +/- u when setting
$no_proxy.
extract_docs will now ignore lines that end with "#nodocs" when
writing devtest.md.

Change-Id: Ibcb6e25c23b4797a21a3b43fedd10e591f68c699
Merge "Ignore some commands when generating devtest.md."