docs/api: add letenv! demo

docs/api.org

@@ -20,6 +20,7 @@ It is integrated into Helpful, in Doom.
     - [[#file-exists-p][file-exists-p!]]
     - [[#lambda][lambda!]]
     - [[#lambda-1][lambda!!]]
+    - [[#letenv][letenv!]]
     - [[#load][load!]]
     - [[#map][map!]]
     - [[#package][package!]]
@@ -268,6 +269,15 @@ Or to create aliases for functions that behave differently:
 ;; its startup visibility settings.
 (fset 'org-reset-global-visibility (lambda!! #'org-global-cycle '(4))
 #+END_SRC
+*** letenv!
+#+BEGIN_SRC elisp
+(letenv! (("SHELL" "/bin/sh"))
+  (shell-command-to-string "echo $SHELL"))
+#+END_SRC
+
+#+RESULTS:
+: "/bin/sh\n"
+
 *** load!
 #+BEGIN_SRC elisp :eval no
 ;;; Lets say we're in ~/.doom.d/config.el