Doc: environment variable

manual/environment-variable.wiki

@@ -0,0 +1,10 @@
+= Environment variables
+
+To resolve an environment variable, js_of_ocaml will perform the
+following steps, in the given order:
+
+- If the variable was set at compile time with the [--setenv VAR] flag, return it.
+- If running nodejs and the variable is set in the process environment ([process.env]), return it.
+- If the variable is set in [globalThis.jsoo_env], return it. This can be used
+  to set an environment variable inside a web-browser.
+- Return Not_found

manual/menu.wiki

@@ -4,6 +4,7 @@
 ==[[library|Library overview]]
 ==[[bindings|Binding a JS library]]
 ==[[rev-bindings|Export OCaml code to JavaScript]]
+==[[environment-variable|Environment variable]]
 ==<<a_api subproject="js_of_ocaml" text="API"|index>>
 
 =Wasm_of_ocaml - Reference Manual