document for E0502 insufficiently general

[yg-i]

Location

Location is E0502.md, the explanation for Error code E0502

https://github.com/rust-lang/rust/blob/master/compiler/rustc_error_codes/src/error_codes/E0502.md

online version: https://doc.rust-lang.org/error_codes/E0502.html

Summary

The doc explains E0502 as follows:

A variable already borrowed as immutable was borrowed as mutable.

However, E0502 is also emitted when a variable already borrowed as mutable was borrowed as immutable. E.g. see line 1 and 13 in this file

1     error[E0502]: cannot borrow `x` as mutable because it is also borrowed as immutable
...
13    error[E0502]: cannot borrow `x` as immutable because it is also borrowed as mutable

So it seems a more general explanation might be advisable that covers (at least) both types of cases.