text on BASE constraints in rq23 (minor substantive)

We had a comment [BASE] and a note to prohibit prefix
re-declarations. The commenter agreed in principle to
the textual addtions in these two paragrphs:

[[
The BASE keyword defines the Base IRI used to resolve relative IRIs
per RFC3986 section 5.1.1, "Base URI Embedded in Content". Section
5.1.2, "Base URI from the Encapsulating Entity" defines how the Base
IRI may come from an encapsulating document, such as a SOAP envelope
with an xml:base directive, or a mime multipart document with a
Content-Location header. The "Retrieval URI" identified in 5.1.3, Base
"URI from the Retrieval URI", is the URL from which a particular
SPARQL query was retrieved. If none of the above specifies the Base
                            ---------------------------------------
URI, the default Base URI (section 5.1.4, "Default Base URI") is used.
----------------------------------------------------------------------
]]

[[
Base URIs declared with the BASE keyword must be absolute URIs. A
---------------------------------------------------------------
prefix declared with the PREFIX keyword may not be re-declared in the
same query. See see section 2.1.1, Syntax of IRI Terms, for a
description of BASE and PREFIX.
]]

The other text in the latter paragraph is freshly invented.

[BASE] http://www.w3.org/mid/7C62EEF9-679B-48BB-AC7E-D6387C7DBB94@tellme.com
-- 
-eric

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