Improve string documentation

[CodesInChaos]

1. Explicitly mention that unpaired surrogates are illegal. This is particularly relevant for Java, C# and javascript programmers, where they are legal in strings.

2. Document supported escape sequences in the documentation for string literals, not just in the grammar specification. Or at least link it from there.

3. Document which encoding is used to store strings (UTF-8? UTF-32?). The documentation points out a 1GB (or is it 1GiB?) limit, but without knowing the encoding that's difficult to interpret.

4. An example at https://www.edgedb.com/docs/edgeql/literals#strings is wrong:

   db> select r'hello\nthere'; # raw string
   {r'hello\\nthere'}
   

   There should be no r in the response line