Add build snippet for non-unique-target-names feature

docs/markdown/snippets/non-unique-target-names.md

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+## Relaxing of target name requirements
+
+In earlier versions of Meson you could only have one target of a given name for each type.
+For example you could not have two executables named `foo`. This requirement is now
+relaxed so that you can have multiple targets with the same name, as long as they are in
+different subdirectories.
+
+Note that projects that have multiple targets with the same name can not be built with
+the `flat` layout or any backend that writes outputs in the same directory.