cbt: improve help descriptions
cbt: improve help descriptions

- Use imperatives (issue #977)
- Add newline to read/lookup help docs
- Clarify optional family/column combination when performing read/lookup

Change-Id: I8276d6301c4d9317177a9e96bc2ef73aa3de6a4c
Reviewed-on: https://code-review.googlesource.com/28570
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go