x/tools/gopls: inconsistent documentation for builtin `error.Error`

[skewb1k]

Currently, error.Error appears to be the only symbol without a doc comment in builtin.go. This causes inconsistency in how gopls behave. Specifically, the signatureHelp method returns a hardcoded string, while other methods like hover and completion show no documentation at all.

[skewb1k]

The solution I see is to add a doc comment in builtin.go and adapt gopls to use it. The starting point for the doc text could be the string already used in signatureHelp:

Error returns the error message.

Are there any pitfalls or concerns I might be missing? I’d appreciate feedback on this and would be glad to help with the implementation.