strconv: document parsing of leading +, and interaction between sign and base prefix

[neild]

The strconv.ParseInt documentation does not state that a leading + is permitted in the input string. This seems worth mentioning.

The documentation states that the input string's prefix indicates the base when the base argument is 0, but does not state that the base prefix must follow the sign when present. (e.g., "-0xf", not "0x-f".) "The string's prefix" is not quite accurate; perhaps this should state "the string's prefix following the sign".

Possibly strconv.ParseUint should explicitly document that it does not accept a leading +.

Also, we don't seem to have any test coverage for leading + characters in ParseInt/ParseUint input.

[gopherbot]

Change https://golang.org/cl/325875 mentions this issue: strconv: document parsing of leading +/-