cloud: improve package doc
cloud: improve package doc

Responses to Ahmet's comments on the already submitted https://code-review.googlesource.com/c/gocloud/+/26310.

Change-Id: I2cca196f0ca04ad5b1b3e349bd168f3af44c266a
Reviewed-on: https://code-review.googlesource.com/26350
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Ahmet Alp Balkan <c786c8527fd6be7431bb94ca48b6f76cc06787cc@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