Rewrite of build target topic
Rewrite of build target topic

Summary:
- Generally improve clarity and readability.
- Note that the build-file name is not used in a build target...which is non-intuitive to people starting with Buck.
- Fix up the regex for a fully-qualified build target.
- Speak to the use of //cell// in the regex and give a reference to where we describe cells in the docs.
- Make the distinction between //full-qualified// build targets and //relative// build targets more clear.
- Improve the formatting of the example.
- Improve the **See also** section, and cross-reference `aliases`.

Reviewed By: bobyangyf

fbshipit-source-id: f2bb17a13e

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