Add some inline documentation for the torbrowser build process

I've added two small comments in hope to make some steps of the build process easier to understand when reading the Makefile.

The commits are at git://github.com/flavioamieiro/torbrowser.git in the 'inline_documentation' branch (https://github.com/flavioamieiro/torbrowser/tree/inline_documentation)

Trac:
Username: amieiro

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