Add comment clarifying presence of empty eval function

share/functions/eval.fish

@@ -1,3 +1,12 @@
-function eval
+# This empty function is required as while `eval` is implemented internally as
+# a decorator, unlike other decorators such as `command` or `builtin`, it is
+# *not* stripped from the statement by the parser before execution to internal
+# quirks in how statements are handled; the presence of this function allows
+# constructs such as command substitution to be used in the head of an
+# evaluated statement.
+#
+# The function defined below is only executed if a bare `eval` is executed (with
+# no arguments), in all other cases it is bypassed entirely.
 
+function eval
 end