Document cargo limitation w/ workspaces & configs

This behavior is described in this bug:
#2930

I think we should document limitations of the tool which exist at present if they do not bind our hands in the future.
If this bug is fixed, this note should be removed.

Author: iankronquist

src/doc/src/reference/config.md

@@ -35,6 +35,16 @@ home directory is the lowest priority. Arrays will be joined together.
 > and is the preferred form. If both files exist, Cargo will use the file
 > without the extension.
 
+
+> **Note:** At present, when being invoked from a workspace, Cargo does not read
+> config files from crates within the workspace. i.e. if a workspace has two
+> crates in it, named `/projects/foo/bar/baz/mylib` and
+> `/projects/foo/bar/baz/mybin`, and there are Cargo configs at
+> `/projects/foo/bar/baz/mylib/.cargo/config.toml`
+> and `/projects/foo/bar/baz/mybin/.cargo/config.toml`, Cargo does not read
+> those configuration files if it is invoked from the workspace root
+> (`/projects/foo/bar/baz/`).
+
 ### Configuration format
 
 Configuration files are written in the [TOML format][toml] (like the