Queries: Expands $clauses array structure explanation in filter docblock

[hellofromtonya]

For WP_Comment_Query, WP_Network_Query, and WP_Site_Query, expands the $clauses array structure explanation in filter DocBlock.

Benefits:

• Consistency with WP_Query and WP_Term_Query.
• Helps to improve understanding.
• Should help to avoid confusion of the purpose for pieces and clauses.

Follow-up to r52974, r53375, and r53376.

Trac ticket: https://core.trac.wordpress.org/ticket/61307

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[hellofromtonya]

Committed via https://core.trac.wordpress.org/changeset/58454.