minor rewrite: move 'SRCS' closer to antecedent; replace 'like' with 'such as'
minor rewrite: move 'SRCS' closer to antecedent; replace 'like' with 'such as'

Summary:
Rewrite the description for `environment_expansion_separator`.

This parameter--and its description--appears in all the genrules. It is *transcluded* into all of these topics from a single source in __genrule_common.soy.

This change is part of a larger effort to rehabilitate our genrules documentation.

{F123656843}

{F123656848}

Reviewed By: styurin, ttsugriy

fbshipit-source-id: a362b57

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