Make PDBGEN documentation clearer.

Tell the developer not to run pdbgen.pl manually, but make.
Describe the destdir environment variable.
Update the /lib directory to the current /libgimp directory.
README_NEW_PDB_PROC: correct spelling.

tools/pdbgen/README

@@ -2,25 +2,35 @@ Some mostly unfinished docs are here.
 
 -Yosh
 
+This document describes the tool PDBGEN.
+
+If you added or modified .pdb files do not run this tool manually but
+run make instead! It will call pdbgen.pl then to generate the files
+into the right output directories.
+
 PDBGEN
 ------------------
 
 What is this?
-It's a tool to automate much of the drudge work of making PDB interfaces
+PDBGEN is a tool to automate much of the drudge work of making PDB interfaces
 to GIMP internals. Right now, it generates PDB description records,
 argument marshallers (with sanity checking) for the app side, as well
 as libgimp wrappers for C plugins. It's written so that extending it
 to provide support for CORBA and other languages suited to static
 autogeneration.
 
-How to invoke pdbgen from the command line:
-Change into the ./tools/pdbgen directory
-  $ ./pdbgen.pl DIRNAME
-where DIRNAME is either "lib" or "app", depending on which set of
-files you want to generate.  The files are written to ./app or ./lib
-in the ./tools/pdbgen directory.  Up to you to diff the file you
-changed and when you're happy copy it into the actual ./app/ or ./lib/
-directory where it gets built.
+Invoking PDBGEN from the command line:
+1. Change into the ./tools/pdbgen directory.
+2. $ ./pdbgen.pl DIRNAME
+where DIRNAME is either "lib" or "app", depending on which set of files
+you want to generate. The files are written into $destdir/app or $destdir/libgimp.
+$destdir is the environment variable destdir. If it's not set,
+then it's the ./tools/pdbgen directory. Make sure the directories
+$destdir/app and $destdir/libgimp already exist and you have write permissions.
+Otherwise the code generator will fail and exit.
+It's up to you to diff the file you changed. When you're happy with
+the generated file, copy it into the actual ./app/ or ./libgimp/ directory
+where it finally gets built.
 
 Anatomy of a PDB descriptor:
 PDB descriptors are Perl code. You define a subroutine, which corresponds

tools/pdbgen/README_NEW_PDB_PROC

@@ -482,7 +482,7 @@ you'll be able to find these lines:
 As you can see from the code above, the +controlpoints+ argument starts
 just as a pointer to a double (array) - you have to do the allocation of
 the array yourself. However, if we would specify an array as an input
-argument, then the pointer will point to it's beginning.
+argument, then the pointer will point to its beginning.
 
 Summary
 -------