Improve clarity of error messages / docs w.r.t. "BUILD" vs "BUILD.bazel"

[brandjon]

Motivation is improving error messages like in #16207, so that users of repos that follow the "BUILD.bazel" convention aren't misled into thinking they need to create a file named "BUILD". (We also have the opposite problem within Google: We wouldn't necessarily want to direct users to create "BUILD.bazel" files when our monorepo exclusively uses "BUILD".)

We could pedantically mention "BUILD or BUILD.bazel" everywhere in errors an documentation, but that could get cumbersome and probably isn't worth its weight to the reader. Alternatively, we could just decide to only ever call them by one name or the other, and ensure the glossary or other concept documentation explains that this is an abbreviated form.