Add descriptions to impl tags within the flags module

armi/reactor/flags.py

@@ -260,6 +260,17 @@ def fromString(cls, typeSpec):
         .. impl:: Retrieve flag from a string.
             :id: I_ARMI_FLAG_TO_STR0
             :implements: R_ARMI_FLAG_TO_STR
+
+            For a string passed as ``typeSpec``, first converts the whole string
+            to uppercase.
+            Then tries to parse the string for any special phrases, as defined
+            in the module dictionary ``_CONVERSIONS``, and converts those phrases
+            to flags directly.
+
+            Then splits the remaining string into separate words based on the presence
+            of spaces. Looping over each of the words, any numbers are stripped out
+            and the remaining string is matched up to any class attribute names.
+            If any matches are found these are returned as flags.
         """
 
         def updateMethod(typeSpec):
@@ -281,6 +292,12 @@ def toString(cls, typeSpec):
         .. impl:: Convert a flag to string.
             :id: I_ARMI_FLAG_TO_STR1
             :implements: R_ARMI_FLAG_TO_STR
+
+            This converts the representation of a bunch of flags from ``typeSpec``,
+            which might look like ``Flags.A|B``,
+            into a string with spaces in between the flag names, which would look
+            like  ``'A B'``. This is done via nesting string splitting and replacement
+            actions.
         """
         return str(typeSpec).split("{}.".format(cls.__name__))[1].replace("|", " ")