mkalldocs.sh required a Unix shell, making it less accessible for
contributors on Windows. It also used a substantially different
codepath to regenerate the file than the one used to check the file
for staleness, making failures in TestDocsUpToDate more complex to
diagnose.
We can solve both of those problems by using the same technique as in
checkScriptReadme: use the test itself as the generator to update the
file. The test is already written in Go, the test binary already knows
how to mimic the 'go' command, and this approach brings the difference
between the test and the generator down to a single flag check.