Fix uses of 'nil' and 'non-nil' in manuals and a few more minor issues

Fix uses of 'nil' and 'non-nil' in manuals and a few more minor issues

* doc/emacs/building.texi (Grep Searching): Fix doc of
'grep-save-buffers'.
(Drag and Drop): Reorder paragraphs.  Fix doc of
'mouse-drag-and-drop-region'.
* doc/emacs/frames.texi (Word and Line Mouse):
* doc/emacs/search.texi (Other Repeating Search):
* doc/lispref/compile.texi (Compilation Functions):
* doc/lispref/files.texi (Directory Names):
* doc/lispref/functions.texi (Advising Named Functions):
* doc/lispref/keymaps.texi (Controlling Active Maps):
* doc/lispref/lists.texi (Association Lists):
* doc/lispref/windows.texi (Quitting Windows): Fix uses of
'non-nil' and 'nil'.