manual: Expand the definition of symlinks and disambiguate "target"

Target can have two opposing meanings:

1. the target directory where symlinks are managed by Stow, and
2. the destinations of those symlinks

So try to move away from this by using the word "destination" for
symlinks.

doc/stow.texi

@@ -258,15 +258,50 @@ target directory, @file{/usr/local/stow} is the stow directory,
 @file{/usr/local/stow/perl} is the package directory, and
 @file{bin/perl} within is part of the installation image.
 
+@anchor{symlink}
 @cindex symlink
+@cindex symlink source
+@cindex symlink destination
 @cindex relative symlink
 @cindex absolute symlink
-A @dfn{symlink} is a symbolic link.  A symlink can be @dfn{relative} or
-@dfn{absolute}.  An absolute symlink names a full path; that is, one
-starting from @file{/}.  A relative symlink names a relative path; that
-is, one not starting from @file{/}.  The target of a relative symlink is
-computed starting from the symlink's own directory.  Stow only
-creates relative symlinks.
+A @dfn{symlink} is a symbolic link, i.e. an entry on the filesystem
+whose path is sometimes called the @dfn{symlink source}, which points to
+another location on the filesystem called the @dfn{symlink destination}.
+There is no guarantee that the destination actually exists.
+
+In general, symlinks can be @dfn{relative} or @dfn{absolute}.  A symlink
+is absolute when the destination names a full path; that is, one
+starting from @file{/}.  A symlink is relative when the destination
+names a relative path; that is, one not starting from @file{/}.  The
+destination of a relative symlink is computed starting from the
+symlink's own directory, i.e. the directory containing the symlink
+source.
+
+@quotation Note
+Stow only creates symlinks within the target directory which point to
+locations @emph{outside} the target directory and inside the stow
+directory.
+
+Consequently, we avoid referring to symlink destinations as symlink
+@emph{targets}, since this would result in the word ``target'' having
+two different meanings:
+
+@enumerate
+
+@item
+the target directory, i.e.@: the directory into which Stow targets
+installation, where symlinks are managed by Stow, and
+
+@item
+the destinations of those symlinks.
+
+@end enumerate
+
+If we did not avoid the second meaning of ``target'', then it would lead
+to confusing language, such as describing Stow as installing symlinks
+into the target directory which point to targets @emph{outside} the
+target directory.
+@end quotation
 
 @c ===========================================================================
 @node Invoking Stow, Ignore Lists, Terminology, Top