doc: improve Sign/Verify examples and docs

• Use complete examples that can be pasted and run as-is
• Move note about algorithm to the functions it applies to
• Uncapitalize inconsistence use of "Class"
• Use both EC and RSA keys in the examples
• Note that hash and digest are two names for the same algorithms

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines