Category membership, without arguments

[nthiery]

Currently one can do:

    sage: F = FreeModule(QQ,3)
    sage: F in VectorSpaces(QQ)
    True

This patch implements:

    sage: F in VectorSpaces
    True

(and does a couple closeby minor improvements to the doc)

CC: @sagetrac-sage-combinat

Component: categories

Keywords: Cernay2012

Author: Nicolas M. Thiéry

Reviewer: Florent Hivert

Merged: sage-5.0.beta6

Issue created by migration from https://trac.sagemath.org/ticket/9469

[nthiery]

Description changed:

--- 
+++ 
@@ -12,3 +12,5 @@
     sage: F in VectorSpaces
     True
 ```
+
+(and does a couple closeby minor improvements to the doc)

[nthiery]

Changed keywords from none to Cernay2012

[nthiery]

comment:4

Patch reviewed by Florent on the Sage-Combinat queue. Positive review on his behalf.