Merge from origin/emacs-26

5381c70b7a In windows.texi mention special splitting behavior of side...
46d62b9f36 * lisp/gnus/gnus-group.el (gnus-group-mode-map): Bind [fol...
1ef1dbdbc5 Avoid compilation warning in xterm.c
5cf3c26f2d * README: Document all the top-level directories.  (Bug#29...
e9ea2065a8 Don't enable erc modules on simply loading erc.el

README

@@ -72,6 +72,8 @@ There are several subdirectories:
 'lib'       holds source code for libraries used by Emacs and its utilities
 'lib-src'   holds the source code for some utility programs for use by or
             with Emacs, like movemail and etags.
+'lwlib'     holds the sources of the Lucid Widget Library used on X.
+'oldXMenu'  source files from X11R2 XMenu library, used in non-toolkit builds.
 'etc'       holds miscellaneous architecture-independent data files Emacs
             uses, like the tutorial text and tool bar images.
             The contents of the 'lisp', 'leim', 'info', and 'doc'
@@ -89,6 +91,10 @@ There are several subdirectories:
             Nextstep port of Emacs, for GNUstep and macOS Cocoa.
 'nt'        holds code and documentation for building Emacs on MS-Windows.
 'test'      holds tests for various aspects of Emacs's functionality.
+'modules'   holds the modhelp.py helper script.
+'admin'     holds files used by Emacs developers, and Unicode data files.
+'build-aux' holds auxiliary files used during the build.
+'m4'        holds Autoconf macros used for generating the configure script.
 
    Building Emacs on non-Posix platforms requires tools that aren't part
 of the standard distribution of the OS.  The platform-specific README