Clarify documentation for --help output in shell scripts

[jotaen4tinypilot]

In our style guide, we have an example that demonstrates the structure/format of the --help text output. We could make this it’s own section instead of having it implicit in the “Parsing Command Line Flags” section, and also elaborate a bit more. There are also some details missing, like how to declare optional flags.

We could also document in which situations we print the full help text. Lately, we’ve been starting to always print the full help text for every input error, e.g. for absent mandatory flags, or for malformed flags. Especially if the help text is long, I found that a little hard to work with, though, because the actual specific error message can get buried, and might be easy to miss. Example: this is the full error output of the publish-release script, if you forget to specify the $GITHUB_TOKEN env variable; only the first of all 52 lines is actually relevant/specific to the error.

Related/Origin #19.