Document that required text parameters need a validator

[bernt-matthias]

Just stumbled over #15491 and found that it deserves a bit more prominent docs.

In my experiments, I found that tools with mandatory text parameters (without validator) can be executed and the parameter is still treated as "" in cheetah. Is the latter a bug? When reading #15491 I got the impression that it should be None.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[mvdbeek]

Could you also mention this in https://docs.galaxyproject.org/en/latest/dev/schema.html#id37 ?

[github-actions]

This PR was merged without a "kind/" label, please correct.