fix: Comparison of optionals and type conversion clarification

[novusnota]

• Made comparison of optionals much more visible
• Clarified implicit conversions for Cells and Slices for == and !=
• A couple of fixes of past typos
• Many smaller enhancements

Closes #173 (clarification of statements on implicit comparison) Closes #174 (better visibility of examples with comparing optionals)

[anton-trunov]

Actually, this duplication of descriptions for != and == should be fixed. I think it makes sense to merge these two subsections together

[novusnota]

Actually, this duplication of descriptions for != and == should be fixed. I think it makes sense to merge these two subsections together

Same can be said for > and <, >= and <=, + and -, etc. :)

[anton-trunov]

Same can be said for > and <, >= and <=, + and -, etc. :)

not really -- those are a lot less polymorphic than == and !=

[anton-trunov]

basically, I'm not a fan of maintaining two non-trivial lists of types for == and !=

[novusnota]

• Applied the suggestion for == and !=
• Made the lists of available application types complete and exhaustive (although didn't use T?, but used "Optionals and null value" instead)