Modernize explanation of `build.rs`

Modernize explanation of `build.rs`

After some recent head-banging, it was explained to me that `"build.rs"` is a special file name, and that Cargo will build such a file even if you do not expect it to, for example if `Cargo.toml` contains the commented out line

```
#build = "build.rs"
```

Some experimentation lead me to the described behavior, but if it misses any subtle details please feel free to correct. The old text is incorrect, though.