mm-inline-large-images-proportion doc clarification

* lisp/gnus/mm-view.el (mm-inline-large-images-proportion): Doc
clarification.
1 job for master in 90 minutes and 2 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #3314
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  ELC      eshell/esh-opt.elc
ELC eshell/esh-proc.elc
ELC eshell/esh-util.elc
ELC eshell/esh-var.elc
ELC eshell/eshell.elc
ELC gnus/canlock.elc
ELC gnus/deuglify.elc
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds