Improve doc strings of 'append-to-buffer' and friends

* lisp/simple.el (append-to-buffer, prepend-to-buffer)
(copy-to-buffer): Doc fixes.
1 job for emacs-26 in 25 minutes and 38 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #2939
test

00:25:38

 
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'
make[1]: *** [check] Error 2
Makefile:255: recipe for target 'check' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:951: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1