Fix the recommendations for conversion of C string constants.

Fix the recommendations for conversion of C string constants.

* doc/ref/api-data.texi (Conversion to/from C, Symbol Primitives,
  Keyword Procedures): Fix the recommendations for conversion of C
  string constants.  The encoding of the source code is irrelevant in
  modern compilers.  What is relevant is the execution character set.
  Both GCC and clang use UTF-8 by default, so recommend the
  'scm_from_utf8_*' functions when the argument is a C string constant.

  Also fix the broken texinfo markup for 'scm_from_*_symbol' that
  prevented them from being listed in the index.