cli: improve add command help (#9230)

(cherry picked from commit 4c24022)

src/poetry/console/commands/add.py

@@ -16,7 +16,7 @@
 
 class AddCommand(InstallerCommand, InitCommand):
     name = "add"
-    description = "Adds a new dependency to <comment>pyproject.toml</>."
+    description = "Adds a new dependency to <comment>pyproject.toml</> and installs it."
 
     arguments = [argument("name", "The packages to add.", multiple=True)]
     options = [