BUG: site.cfg.example should describe the actual BLAS detection process

Instead of gh-2751 (`ENH: Respect blas_opt and lapack_opt groups in site.cfg`), this pull request updates `site.cfg.example` to describe the actual detection process for the BLAS & LAPACK libraries.

I don't expect that this pull request will be well received, but it should be useful for discussion as I believe it documents the actual state of affairs in the `site.cfg` file.

Primary changes:
* Remove deprecation of `[atlas]` section.
* Highlight need to "zero-out" certain sections to avoid them in the autodetection process.
* Point out that `_dotblas` won't be built if using the `[blas]` section.

I haven't thoroughly proofread this. Send any corrections my way and I'll happily update the pull request.