Fixed inconsistent usage of keep_local and local_only kwarg in Help() function, its implementation and docs

[hedger]

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt (and read the README.rst)
• I have updated the appropriate documentation

[bdbaddog]

Not sure why you beleive this change is needed?
local_only, means only the local addoptions args are in the help.. what's the concern here?

[hedger]

@bdbaddog the concern is that documentation is inconsistent with actual behavior.

Currently, the docs mention the following prototype for the Help() function:

env.Help(text, append=False, local_only=False)

While actual implementation in

scons/SCons/Script/__init__.py

Line 254 in 4f3e302

def HelpFunction(text, append: bool = False, keep_local: bool = False) -> None:

uses the keep_local kwarg name.

I made the documentation consistent with the actual implementation - in order to not break any third-party code that relies on existing behavior. I also renamed the kwarg inside the implementation to avoid confusion.