This change causes Starlark, like Go, to reject backslashes that
are not part of an escape sequence. Previously they were treated
literally, so \ ( would encode a two-character string.

Many programs rely on this, especially for regular expressions and
shell commands, and will be broken by this change, but the fix is simple:
double each errant backslash.

Python does not yet enforce this behavior, but since 3.6
has emitted a deprecation warning for it.

Also, document string escapes.

Related issues:
- Google issue b/34519173: "bazel: Forbid undefined escape sequences in strings"
- bazelbuild/starlark#38: Starlark spec: String escapes
- bazelbuild/buildtools#688: Bazel: Fix string escapes
- bazelbuild/bazel#8380: Bazel incompatible_restrict_string_escapes: Restrict string escapes