Tiny improvement of documentation of major mode conventions

* doc/lispref/modes.texi (Major Mode Conventions): More
accurate advice regarding customization of 'C-M-a' by major
modes.
1 job for emacs-26 in 4 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #2046
test

00:00:04

 
Name Stage Failure
failed
test Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)