cmd/cue: `help injection` should say how to enable injection

[rogpeppe]

What version of CUE are you using (cue version)?

$ cue version
v0.9.1

Does this issue reproduce with the latest stable release?

Yes

What did you do?

cue help injection

What did you expect to see?

Complete-enough explanation of injection that I can run a few small examples to try it out.

What did you see instead?

The injection mechanism allows for the injection of system variables: when variable injection is enabled, tags of the form [...] will inject the named variable (here cwd) into the tag.

Nowhere does it seem to say how variable injection is enabled (with flag -T or --inject-vars). It probably should.