Document default entrypoint behavior and clean up formatting
Document default entrypoint behavior and clean up formatting

Also rename references from generic builder to general.

Fixes #69

PiperOrigin-RevId: 325532902
Change-Id: I1dff683aaa9ed305ad4e38331d164273b4fa1f1a

GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks,GoogleCloudPlatform/buildpacks