update documentation related to PKG_* to include 'VERSION'

[KtorZ]

Hello!

I've only experienced this for PKG_PATCH which would lead to the following warning:

At least one environment variable looks like an override-variable but was ignored by built: `BUILT_OVERRIDE_my_pkg_PKG_PATCH`. Typo?"

Using BUILT_OVERRIDE_my_pkg_PKG_VERSION_PATCH fixes the problem; and I assumed other fields would follow the same rule.

[lukaslueg]

thanks