Update description for #be_boolean matcher.

Current spec status:
- 2.0: 393 examples, 0 failures in 0.11576 seconds.
- 2.1: 458 examples, 0 failures in 0.10224 seconds.
Update description for #construct matcher.

Current spec status:
- 2.0: 402 examples, 0 failures in 0.1054 seconds.
- 2.1: 467 examples, 0 failures in 0.10252 seconds.