nix(docs): print message to console on script runs

It's not immediately visible that these scripts are run when
doing `postgrest-docs-check`.

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>
(cherry picked from commit e1d547623e)

nix/tools/docs.nix

@@ -122,6 +122,8 @@ let
         workingDir = "/docs";
       }
       ''
+        echo "Checking spelling mistakes..."
+
         export LC_ALL=C
 
         FILES=$(find . -type f -iname '*.rst' | tr '\n' ' ')
@@ -144,6 +146,8 @@ let
         workingDir = "/docs";
       }
       ''
+        echo "Detecting obsolete dictionary entries..."
+
         export LC_ALL=C
 
         FILES=$(find . -type f -iname '*.rst' | tr '\n' ' ')