Correctly typeset nil and t in texinfo

* doc/emacs/package.texi (Fetching Package Sources):
* doc/lispref/display.texi (Image Descriptors):
* doc/lispref/windows.texi (Buffer Display Action Alists):
* doc/misc/gnus-faq.texi (FAQ 5-4): Correctly typeset nil and t.

Author: skangas

doc/emacs/package.texi

@@ -693,9 +693,9 @@ an Org file.
 
 @item :make
 A string or list of strings providing the target or targets defined in
-the repository Makefile which should run before building the Info
-file.  Only takes effect when @code{package-vc-allow-build-commands}
-is non-nil.
+the repository Makefile which should run before building the Info file.
+Only takes effect when @code{package-vc-allow-build-commands} is
+non-@code{nil}.
 
 @item :shell-command
 A string providing the shell command to run before building the Info

doc/lispref/display.texi

@@ -6177,7 +6177,7 @@ or @code{:flip} is changed.
 If @code{:original-map} is not specified when creating an image with
 @code{create-image}, it will be computed based on the supplied
 @code{:map}, as well as any of @code{:scale}, @code{:rotation}, or
-@code{:flip} which are non-nil.
+@code{:flip} which are non-@code{nil}.
 
 Conversely, if @code{:original-map} is specified but @code{:map} is not,
 @code{:map} will be computed based on @code{:original-map},

doc/lispref/windows.texi

@@ -3312,12 +3312,11 @@ only addressee.
 
 @vindex pop-up-frames@r{, a buffer display action alist entry}
 @item pop-up-frames
-The value controls whether @code{display-buffer} may display buffers
-by making new frames.  It has the same meaning as the
-@code{pop-up-frames} variable and takes precedence over it when present.
-Its main intended purpose is to override a non-nil value of the
-variable for particular buffers which the user prefers to keep
-in the selected frame.
+The value controls whether @code{display-buffer} may display buffers by
+making new frames.  It has the same meaning as the @code{pop-up-frames}
+variable and takes precedence over it when present.  Its main intended
+purpose is to override a non-@code{nil} value of the variable for
+particular buffers which the user prefers to keep in the selected frame.
 
 @vindex parent-frame@r{, a buffer display action alist entry}
 @item parent-frame

doc/misc/gnus-faq.texi

@@ -1173,12 +1173,11 @@ to postings you send to the gmane hierarchy, use
 send to groups containing the string binaries in their
 name etc.
 
-You can instead of specifying a regexp specify a function
-which is evaluated, only if it returns true, the
-corresponding settings take effect.  Two interesting
-candidates for this are message-news-p which returns t if
-the current Group is a newsgroup and the corresponding
-message-mail-p.
+You can instead of specifying a regexp specify a function which is
+evaluated, only if it returns true, the corresponding settings take
+effect.  Two interesting candidates for this are @code{message-news-p}
+which returns @code{t} if the current Group is a newsgroup and the
+corresponding @code{message-mail-p}.
 
 Note that all forms that match are applied, that means in
 the example below, when I post to