Clarify purpose of cxx_genrule and explain where the rule gets the information it exposes to the shell command
Clarify purpose of cxx_genrule and explain where the rule gets the information it exposes to the shell command

Summary:
This diff clarifies the purpose of cxx_genrule and describes where it sources the information it provides.

- Rewrite the introduction to explain what `cxx_genrule` enables and
  also to explain where the information, exposed by `cxx_genrule`, about
  the buck build environment is sourced.

- Explain that the information about the build environment is avaiable
  only to the shell commands specified in the `cxx_genrule`.

**NOTE** The aberrant text in the highlighted box below is fixed by D7620192, which this diff depends on.

{F123660435}

Reviewed By: ttsugriy

fbshipit-source-id: 0470181

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