Improve versioning algorithm description

[Ralf Wildenhues]

The current text in the "Updating version info" node is seen as hard to
understand by some users, IIRC we've had a few reports about this in the
past.  Richard made me try to reformulate it now.

What do you think about this additional explanation?
OK to apply (and ok to add you to THANKS, Richard)?

Thanks,
Ralf

    Improve versioning algorightm documentation.

    * doc/libtool.texi (Updating version info): Repeat the
    algorithms in different, hopefully simpler terms.
    * THANKS: Update.
    Report by Richard B. Kreckel and others, several times.

diff --git a/doc/libtool.texi b/doc/libtool.texi
index 08a44c4..cb6ec80 100644
--- a/doc/libtool.texi
+++ b/doc/libtool.texi
@@ -2879,6 +2879,37 @@ Instead, use the @option{-release} flag (@pxref{Release 
numbers}), but be
 warned that every release of your package will not be binary compatible
 with any other release.
 
+The following explanation may help to understand the above rules a bit
+better: consider that there are three possible kinds of reactions from
+users of your library to changes in a shared library:
+
address@hidden 1
address@hidden
+Programs using the previous version may use the new version as
+drop-in replacement, and programs using the new version can also work
+with the previous one.  In other words, no recompiling nor relinking
+is needed.  In this case, bump @var{revision} only, don't touch
address@hidden nor @var{age}.
+
address@hidden
+Programs using the previous version may use the new version as
+drop-in replacement, but programs using the new version may use APIs not
+present in the previous one.  In other words, a program linking against
+the new version may fail with ``unresolved symbols'' if linking against
+the old version at runtime: set @var{revision} to 0, bump @var{current}
+and @var{age}.
+
address@hidden
+Programs may need to be changed, recompiled, relinked in order to use
+the new version.  Bump @var{current}, set @var{revision} and @var{age}
+to 0.
address@hidden enumerate
+
address@hidden
+In the above description, @emph{programs} using the library in question
+may also be replaced by other libraries using it.
+
+
 @node Release numbers
 @section Managing release information