mention 'Imports: methods' alternative, with caveats (PR#18540)

git-svn-id: https://svn.r-project.org/R/trunk@84688 00db46b3-68df-0310-9c12-caf00c1e9a41

doc/manual/R-exts.texi

@@ -4690,8 +4690,12 @@ standard function @code{c} in @pkg{base}.
 
 Some additional steps are needed for packages which make use of formal
 (S4-style) classes and methods (unless these are purely used
-internally). The package should have @code{Depends: methods} in its
-@file{DESCRIPTION} and @code{import(methods)} or
+internally). The package should have
+@code{Depends: methods}@footnote{@code{Imports: methods} may suffice,
+but package code is little exercised without the @pkg{methods} package
+on the search path and may not be fully robust to this scenario.
+@c https://bugs.R-project.org/show_bug.cgi?id=18540#c10
+} in its @file{DESCRIPTION} and @code{import(methods)} or
 @code{importFrom(methods, ...)} plus any classes and methods which are
 to be exported need to be declared in the @file{NAMESPACE} file. For
 example, the @pkg{stats4} package has