[populate] fix documentation

This patch fixes the existing documentation:
- document -l and -L
- use syntax more in line with the help entry

docs/overview.txt

@@ -412,6 +412,9 @@ eg. /your/root. This directory is the /image/ of what would be in the root file
 system of your target, and will contain all that your programs/packages have
 installed.
 
+The 'populate' script |
+----------------------+
+
 When your root directory is ready, it is still missing some important bits: the
 toolchain's libraries. To populate your root directory with those libs, just
 run:
@@ -425,23 +428,32 @@ contains only those things you install in it.
 You can then use /your/root-populated to build up your file system image, a
 tarball, or to NFS-mount it from your target, or whatever you need.
 
-populate accepts the following options:
+The populate script accepts the following options:
 
- -s [src_dir]
+ -s src_dir
-    Use 'src_dir' as the 'source', un-populated root directory
+    Use 'src_dir' as the un-populated root directory.
 
- -d [dst_dir]
+ -d dst_dir
-    Put the 'destination', populated root directory in 'dst_dir'
+    Put the populated root directory in 'dst_dir'.
+
+ -l lib1 [...]
+    Always add specified libraries.
+
+ -L file
+    Always add libraries listed in 'file'.
 
  -f
-    Remove 'dst_dir' if it previously existed
+    Remove 'dst_dir' if it previously existed; continue even if any library
+    specified with -l or -L is missing.
 
  -v
     Be verbose, and tell what's going on (you can see exactly where libs are
     coming from).
 
  -h
-    Print the help
+    Print the help.
+
+See 'your-target-tuple-populate -h' for more information on the options.
 
 ___________________
                   /