Clarify consequences of mismatching Flymake diagnostic buffers

Fix bug#34418.

* doc/misc/flymake.texi (Flymake utility functions): Add note to
flymake-make-diagnostic.

* lisp/progmodes/flymake.el (flymake--handle-report): Discard
diagnostics from other buffers.

1 files changed, 3 insertions, 1 deletions

diff --git a/doc/misc/flymake.texi b/doc/misc/flymake.texi
index ebb89c32036..cedf1d84f90 100644
--- a/doc/misc/flymake.texi
+++ b/doc/misc/flymake.texi
@@ -543,7 +543,9 @@ by calling the function @code{flymake-make-diagnostic}.
 Make a Flymake diagnostic for @var{buffer}'s region from @var{beg} to
 @var{end}.  @var{type} is a diagnostic symbol (@pxref{Flymake error
 types}), and @var{text} is a description of the problem detected in
-this region.
+this region.  Currently, it is unspecified behaviour to make
+diagnostics for buffers other than the buffer that the Flymake backend
+is responsible for.
 @end deffn
 
 @cindex access diagnostic object