Clarify when the identifier map and the counter are reset

This reverts commit 0f13c832a0214b0b01b6c803d74b0dadd69519ba.

See https://github.com/json-ld/json-ld.org/commit/177b5041c9fc145d84ddb307b8f11c86c805fca2#commitcomment-3376131

/cc @dlongley @gkellogg