Document all variables available in package definition functions

[nerdvegas]

In a package.py, various different objects (such as env, this) are available depending on whether a package is being built, and where you are in the package definition (ie, in an early bound func, or late bound, or in a commands or commands-like function).

The existing docs do describe what objects are available in what context, but it's hard to track this without the info being provided in one place.

A table should be added showing a matrix of potential cases (eg, in commands, building=true; in early bound func, etc), along with the available objects, and a description of each.

Related - there should also be a table showing which package attributes are available in released packages (as some are stripped according to some rules). Currently this is just a list, but it needs to be updated (eg, I think private_build_requires is actually kept in the released pkg now).

https://github.com/nerdvegas/rez/wiki/Package-Definition-Guide#build-time-package-attributes