tableam: basic documentation.

This adds documentation about the user oriented parts of table access
methods (i.e. the default_table_access_method GUC and the USING clause
for CREATE TABLE etc), adds a basic chapter about the table access
method interface, and adds a note to storage.sgml that it's contents
don't necessarily apply for non-builtin AMs.

Author: Haribabu Kommi and Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

src/include/access/tableam.h

@@ -9,6 +9,9 @@
  *
  * src/include/access/tableam.h
  *
+ * NOTES
+ *		See tableam.sgml for higher level documentation.
+ *
  *-------------------------------------------------------------------------
  */
 #ifndef TABLEAM_H