docs: Documentation for built-in functions in Jenny

[st-pasha]

Description

This PR adds missing documentation for all built-in functions available in Jenny.

In addition:

• The bool() function now behaves the same way as the conversion of arguments for an invocation of a user-defined function;
• Consequently, static sets trueValues/falseValues moved from CommandStorage class into the YarnProject;
• Added some tests for several other functions.

Checklist

• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [-] I have updated/added relevant examples in examples or docs.

Breaking Change?

• No, this PR is not a breaking change.

Related Issues

Closes #2236