minor copyedit. slightly clarify button.active use in docs

BS3 commits 1113224e97844675f92697ae6c6612bb5a2234d4,
488b8dad2b68c2053eb3e1b84ee624d26d8981a3
minor copyedit.

BS3 commit a1b3fb339e911cd0c7d472d40cbca3e01ed221f4