bigtable: document behavior of passing multiple RowFilters
bigtable: document behavior of passing multiple RowFilters

Fixes #263

Change-Id: I25fc2c34f528d177e3c7873b4cf836e6f2a2105c
Reviewed-on: https://code-review.googlesource.com/15771
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