Merge from origin/emacs-27

Merge from origin/emacs-27

1dfc497fac Minor wording change in Introduction to Programming in Ema...
ff09b4eeac Fix 'flymake-show-diagnostics-buffer' when line numbers ar...
63e8d0ea87 Fix last changes describing mail commands
01212a762f Do setup Flymake in file-less Elisp buffers
36873ef2b2 Fix error message for ‘cl-struct-unknown-slot’ (bug#39995)
3f9310b0fe Fix and improve documentation of mail-related features
1482a75efa Fix build failure with Fx_gtk_debug
cf57663f2a Mention jit-lock deferred as an alternative to fast-but-im...

# Conflicts:
#	etc/NEWS