[intro.races] Add index entries for notions of coherence.

source/intro.tex

@@ -1416,12 +1416,14 @@
 \end{note}
 
 \pnum
+\indextext{coherence!write-write}%
 If an operation \placeholder{A} that modifies an atomic object \placeholder{M} happens before
 an operation \placeholder{B} that modifies \placeholder{M}, then \placeholder{A} shall be earlier
 than \placeholder{B} in the modification order of \placeholder{M}. \begin{note} This requirement
 is known as write-write coherence. \end{note}
 
 \pnum
+\indextext{coherence!read-read}%
 If a
 \indextext{value computation}%
 value computation \placeholder{A} of an atomic object \placeholder{M} happens before a
@@ -1434,6 +1436,7 @@
 \begin{note} This requirement is known as read-read coherence. \end{note}
 
 \pnum
+\indextext{coherence!read-write}%
 If a
 \indextext{value computation}%
 value computation \placeholder{A} of an atomic object \placeholder{M} happens before an
@@ -1443,6 +1446,7 @@
 read-write coherence. \end{note}
 
 \pnum
+\indextext{coherence!write-read}%
 If a
 \indextext{side effects}%
 side effect \placeholder{X} on an atomic object \placeholder{M} happens before a value