Update: add documentation about return codes.

diff --git a/documents/return_codes.txt b/documents/return_codes.txt
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+# fss-0002
+
+Implicit Return Types for f_status_t:
+  When f_status_t is used as the return type, there are a common set of status codes that should always be expected as a possible return value.
+  These are implicit expectations even if they are not explicitly documented.
+  Any function that doesn't support these must document the exceptional state.
+
+  F_failure\:
+    This may be returned for any general error.
+    This is often used as the failsafe error code so that an error code of some sort is always returned on error.
+
+  F_parameter\:
+    This is always potentially returned for any function whose parameter requires a validity checked.
+    Much of this functionality can be optionally disabled, but the client using the API need not be aware.
+
+  F_supported_not\:
+    This is used to designate that the function or some part of the function is not supported.
+    This is primarily used by individuals altering the project but maintaing API compatibility.
+    Any function must therefore support returning F_suppoted_not.
+
+    There are some projects that can be enabled/disabled such as f_capability and f_threads.
+    These may explicitly document returning F_supported_not.
+
+    The F_suppoted_not may be used for other purposes as well.
+    @todo Consider replacing this with F_function_unsupported as a special case to allow for F_suppoted_not to be usable for normal purposes.
+          Using F_suppoted_not is currently ambiguous between the function being unsupported and some functionality provided by the function being unsupported.
+
+Common Return Types for f_status_t:
+  There are some f_status_t return types that are not required to be returned but are used quite often that they might seem required.
+
+  F_none\:
+    This general designates that no error occurred.
+    This is a generic status code used all over the place.
+
+  F_memory_not\:
+    This general designates that the operation ran out of memory.
+    This is a generic status code used all over the place.