Document meta.mainProgram

Issue #4498.

src/nix/run.md

@@ -43,9 +43,10 @@ program specified by the app definition.
 
 If *installable* evaluates to a derivation, it will try to execute the
 program `<out>/bin/<name>`, where *out* is the primary output store
-path of the derivation and *name* is the name part of the value of the
-`name` attribute of the derivation (e.g. if `name` is set to
-`hello-1.10`, it will run `$out/bin/hello`).
+path of the derivation and *name* is the `meta.mainProgram` attribute
+of the derivation if it exists, and otherwise the name part of the
+value of the `name` attribute of the derivation (e.g. if `name` is set
+to `hello-1.10`, it will run `$out/bin/hello`).
 
 # Flake output attributes