user's guide: flesh out XOverloadedStrings docs

This commit extends the documentation of the OverloadedStrings extension
with some usage information, in particular suggestions to:

  - use default declarations, such as `default (Text)` or
    `default IsString(Text)` (with the NamedDefaults extension),

  - enable the ExtendedDefaultRules extension to relax the requirement
    that a defaultable type variable must only appear in unary standard
    classes

Fixes #23388