bug#59779: string-equal-ignore-case's docstring is misleading

[Nicolas Graner]

The docstring for string-equal-ignore-case reads:
  Like ‘string-equal’, but case-insensitive.

However, string-equal accepts symbols and strings as arguments, while
string-equal-ignore-case accepts only strings. This can be confusing to
people familiar with string-equal.

Either string-equal-ignore-case should be made to accept symbols as
arguments, or the doc should be clarified.

Nicolas