relocatable: improve documentation

[Akim Demaille]

If we follow blindly the documentation, it won't work, as some variables 
substituted by config.status depend on other variables.

Alternatively, @relocatable_sh@ could define these variables.

commit c3d916e5a4d4da2319c4d4eaf79cdc09fcfd6a36
Author: Akim Demaille <address@hidden>
Date:   Sun Jan 13 19:48:35 2019 +0100

    relocatable: improve documentation
    
    * doc/relocatable-maint.texi (Supporting Relocation): For
    substitutions performed by config.status, we need more variables
    (for instance datarootdir defaults to '${prefix}/share' so we need
    '${share}).

diff --git a/ChangeLog b/ChangeLog
index d49779499..9b7acd9e0 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2019-01-13  Akim Demaille  <address@hidden>
+
+       relocatable: improve documentation.
+       * doc/relocatable-maint.texi (Supporting Relocation): For
+       substitutions performed by config.status, we need more variables
+       (for instance datarootdir defaults to '${prefix}/share' so we need
+       '${share}).
+
 2019-01-13  Akim Demaille  <address@hidden>
 
        backup: update dependencies
diff --git a/doc/relocatable-maint.texi b/doc/relocatable-maint.texi
index d1e7090bc..b95caaf7c 100644
--- a/doc/relocatable-maint.texi
+++ b/doc/relocatable-maint.texi
@@ -122,8 +122,12 @@ shell script that your package installs, add the following:
 
 @smallexample
 @@relocatable_sh@@
+
+prefix="@@prefix@@"
+exec_prefix="@@exec_prefix@@"   # usually needs $prefix.
+datarootdir="@@datarootdir@@"   # usually needs $prefix.
+
 if test "@@RELOCATABLE@@" = yes; then
-  exec_prefix="@@exec_prefix@@"
   bindir="@@bindir@@"
   orig_installdir="$bindir" # see Makefile.am's *_SCRIPTS variables
   func_find_curr_installdir # determine curr_installdir
@@ -140,8 +144,9 @@ else
 fi
 
 # Get some relocated directory names.
-sysconfdir=`relocate "@@sysconfdir@@"`
-some_datadir=`relocate "@@datadir@@/something"`
+sysconfdir=`relocate "@@sysconfdir@@"`          # usually needs $prefix.
+some_datadir=`relocate "@@datadir@@/something"` # usually needs $datarootdir.
+bindir=`relocate "@@bindir@@"`       # usually needs $exec_prefix, hence 
$prefix.
 @end smallexample
 
 You must adapt the definition of @code{orig_installdir}, depending on