[Docs] Document the unique port attribution rule. (#20087)

* Document the unique port attribution rule.

* Remove reference to #19922

* @strega-nil suggestions :D

Co-authored-by: nicole mazzuca <83086508+strega-nil-ms@users.noreply.github.com>

Co-authored-by: nicole mazzuca <83086508+strega-nil-ms@users.noreply.github.com>

docs/maintainers/maintainer-guide.md

@@ -52,6 +52,28 @@ At this time, the following helpers are deprecated:
 Ideally, portfiles should be short, simple, and as declarative as possible.
 Remove any boiler plate comments introduced by the `create` command before submitting a PR.
 
+### Ports must not be path dependent
+
+Ports must not change their behavior based on which ports are already installed in a form that would change which contents that port installs. For example, given:
+
+```
+> vcpkg install a
+> vcpkg install b
+> vcpkg remove a
+```
+
+and
+
+```
+> vcpkg install b
+```
+
+the files installed by `b` must be the same, regardless of influence by the previous installation of `a`. This means that ports must not try to detect whether something is provided in the installed tree by another port before taking some action. A specific and common cause of such "path dependent" behavior is described below in "When defining features, explicitly control dependencies."
+
+### Unique port attribution rule
+
+In the entire vcpkg system, no two ports a user is expected to use concurrently may provide the same file. If a port tries to install a file already provided by another file, installation will fail. If a port wants to use an extremely common name for a header, for example, it should place those headers in a subdirectory rather than in `include`.
+
 ## Features
 
 ### Do not use features to implement alternatives