Add index for "\( in strings" (Bug#25195)

* doc/emacs/programs.texi (Left Margin Paren): Add index for "\( in
strings".
* doc/lispref/positions.texi (List Motion): Add index, and cross
reference.
1 job for emacs-26 in 32 minutes and 39 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #2795
test

00:32:39

 
Name Stage Failure
failed
test Test
make[2]: *** [check-doit] Error 1
Makefile:285: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:255: recipe for target 'check' failed
make[1]: *** [check] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check] Error 2
Makefile:951: recipe for target 'check' failed
ERROR: Job failed: exit code 1