Update doc/pages/Manual.md

Co-authored-by: R. Boujbel <rjbou@ocamlpro.com>

doc/pages/Manual.md

@@ -1112,14 +1112,17 @@ files.
   build, install and remove scripts.
 
     The following environment variables are set by opam (but can be overridden by `build-env`):
-    - `CDPATH=`
-    - `MAKEFLAGS=`
-    - `MAKELEVEL=`
-    - `OPAM_SWITCH_PREFIX=<switch prefix>`
-    - `OPAMSWITCH=<switch name>`
-    - `OPAM_PACKAGE_NAME=<pkg>` (`<pkg>` is the name of the package being built/installed/removed)
-    - `OPAM_PACKAGE_VERSION=<ver>` (`<ver>` is the version of the package being built/installed/removed)
-    - `OPAMCLI=2.0` (since opam 2.1)
+    - can be overridden by `build-env`:
+      - `CDPATH=`
+      - `MAKEFLAGS=`
+      - `MAKELEVEL=`
+      - `OPAM_PACKAGE_NAME=<pkg>` (`<pkg>` is the name of the package being built/installed/removed)
+      - `OPAM_PACKAGE_VERSION=<ver>` (`<ver>` is the version of the package being built/installed/removed)
+      - `OPAMCLI=2.0` (since opam 2.1)
+    - non overwritable:
+      - `OPAMROOT`=<opam root path>`
+      - `OPAMSWITCH=<switch name>`    
+      - `OPAM_SWITCH_PREFIX=<switch prefix>`
     - `TMP` and `TMPDIR` are set by the sandbox script (bubblewrap), but should not be relied on since the sandbox is not used on all platforms and can be disabled by the user.
 
 - <a id="opamsection-extra-sources">`extra-source <string> "{" <url-file> "}"`</a>: