strings: document properties of EqualFold

[dsnet]

It is unclear whether EqualFold implements some common operation defined in the Unicode standard. If so we should document what that is and reference it. It is also unclear what properties of EqualFold are guaranteed. Is it safe to assume that EqualFold holds to the same relations that one would expect from equality?

• Reflexive: strings.EqualFold(a, a)?
• Symmetric: strings.EqualFold(a, b) == strings.EqualFold(b, a)?
• Transitive: if strings.EqualFold(a, b) && strings.EqualFold(b, c), then strings.EqualFold(a, c)?

Similar documentation should be made on bytes.EqualFold

[toothrot]

/cc @griesemer

[griesemer]

Originally written by @rsc ( https://codereview.appspot.com/5143044 ).

[gopherbot]

Change https://golang.org/cl/336431 mentions this issue: strings, bytes: document properties of EqualFold