docs: dev/upgrade: some minor text improvements

docs/dev/upgrade.rst

@@ -6,7 +6,7 @@ Basics
 
 After each sysupgrade (including the initial installation), Gluon will execute all scripts
 under ``/lib/gluon/upgrade``. These scripts' filenames usually begin with a 3-digit number
-specifying the order of execution.
+specifying the order of execution. Note that the script files need to be executable.
 
 To get an overview of the ordering of all scripts of all packages, the helper script ``contrib/lsupgrade.sh``
 in the Gluon repository can be used, which will print all upgrade scripts' filenames and directories. If executed
@@ -19,14 +19,12 @@ Best practices
   by Gluon, e.g. to access the site configuration or edit UCI configuration files.
 
 * Whenever possible, scripts shouldn't check if they are running for the first time, but just edit configuration
-  files to achive a valid configuration (without overwriting configuration changes made by the user where desirable).
+  files to achieve a valid configuration (without overwriting configuration changes made by the user where desirable).
   This allows using the same code to create the initial configuration and upgrade configurations on upgrades.
 
 * If it is unavoidable to run different code during the initial installation, the ``sysconfig.gluon_version`` variable
   can be checked. This variable is ``nil`` during the initial installation and contains the previously install Gluon
-  version otherwise. The package ``gluon-legacy`` (which is responsible for upgrades from the old firmwares of
-  Hamburg/Kiel/Lübeck) uses the special value ``legacy``; other packages should handle this value just as any other
-  string.
+  version otherwise.
 
 Script ordering
 ---------------