mm-inline-large-images doc string clarification

* lisp/gnus/mm-decode.el (mm-inline-large-images): Clarify doc string.
1 job for master in 90 minutes and 3 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #3312
test-all

01:30:03

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  ELC      gnus/nnregistry.elc
ELC gnus/nnrss.elc
ELC gnus/nnspool.elc
ELC gnus/nntp.elc
ELC gnus/nnvirtual.elc
ELC gnus/nnweb.elc
ELC gnus/score-mode.elc
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds