[container.reqmts] Fix cross-references to contiguous container

This edit deserves describing in detail, to establish that it
is purely editorial.

As part of the mission to clean up tables in the standard,
the general container requirements were split into 5 more
focused leaf nodes.  Due to its location in the original
text, the definition for a contiguous container fell into
the subclause for reversible containers, to which it is
entirely unrelated.  There is no requirement that a
contiguous container be reversible, only that it has the
correct category for its iterators.

Meanwhile, all 3 of the existing cross-references point
to the wrong leaf node, that simply provides a key to
the identifiers used throughout the specification of this
clause.

The fix has two parts.  First move the definition of
contiguous container, and a container with special
iterators, into the [container.reqmts] clause where it
best belongs.  This move is appended to the end so that
there can be no ambiguity that any existing text could
be confused with requirements only on contiguous
containers after the edit.  The other part is to fix up
the three cross-references to point to [container.reqmts]
rather than its sibling that has no information on
contiguous containers.

A grep of the .tex source files confirms that these
three references (array,basic_string, and vector) are
the only current uses of contiguous container, even
though basic_stacktrace would also meet the requirements.

Author: AlisdairM

source/containers.tex

@@ -628,6 +628,14 @@
 with value \tcode{a.end()} before the swap will have value \tcode{b.end()} after the
 swap.
 
+\pnum
+A \defnadj{contiguous}{container}
+is a container
+whose member types \tcode{iterator} and \tcode{const_iterator}
+meet the
+\oldconcept{RandomAccessIterator} requirements\iref{random.access.iterators} and
+model \libconcept{contiguous_iterator}\iref{iterator.concept.contiguous}.
+
 \rSec3[container.rev.reqmts]{Reversible container requirements}
 
 % Local command to index names as members of all containers.
@@ -809,14 +817,6 @@
 shall not invalidate iterators to, or change the values of, objects
 within that container.
 
-\pnum
-A \defnadj{contiguous}{container}
-is a container
-whose member types \tcode{iterator} and \tcode{const_iterator}
-meet the
-\oldconcept{RandomAccessIterator} requirements\iref{random.access.iterators} and
-model \libconcept{contiguous_iterator}\iref{iterator.concept.contiguous}.
-
 \rSec3[container.opt.reqmts]{Optional container requirements}
 
 \pnum
@@ -6149,7 +6149,7 @@
 \indextext{\idxcode{array}!contiguous storage}%
 The header \libheader{array} defines a class template for storing fixed-size
 sequences of objects.
-An \tcode{array} is a contiguous container\iref{container.requirements.general}.
+An \tcode{array} is a contiguous container\iref{container.reqmts}.
 An instance of \tcode{array<T, N>} stores \tcode{N} elements of type \tcode{T},
 so that \tcode{size() == N} is an invariant.
 
@@ -8590,7 +8590,7 @@
 of a sequence container, including most of the optional sequence container
 requirements\iref{sequence.reqmts},
 and, for an element type other than \tcode{bool},
-of a contiguous container\iref{container.requirements.general}.
+of a contiguous container\iref{container.reqmts}.
 The exceptions are the
 \tcode{push_front}, \tcode{prepend_range}, \tcode{pop_front}, and \tcode{emplace_front} member functions, which are not
 provided. Descriptions are provided here only for operations on \tcode{vector}

source/strings.tex

@@ -2006,7 +2006,7 @@
 is designated by \tcode{charT}.
 
 \pnum
-A specialization of \tcode{basic_string} is a contiguous container\iref{container.requirements.general}.
+A specialization of \tcode{basic_string} is a contiguous container\iref{container.reqmts}.
 
 \pnum
 In all cases,