Improve docs for picotool load -x (#281)

* Mention flash update boot in load -x description

* Include RAM boot

Author: will-v-pi

README.md

@@ -300,7 +300,8 @@ OPTIONS:
         -v, --verify
             Verify the data was written correctly
         -x, --execute
-            Attempt to execute the downloaded file as a program after the load
+            Perform a bootrom reboot to execute the downloaded file as a program after the load - either a flash update boot for binaries in
+            flash, or a RAM image boot for other binaries 
     File to load from
         <filename>
             The file name

main.cpp

@@ -854,7 +854,7 @@ struct load_command : public cmd {
                 option('N', "--no-overwrite-unsafe").set(settings.load.no_overwrite_force) % "When writing flash data, do not overwrite an existing program in flash. If picotool cannot determine the size/presence of the program in flash, the load continues anyway" +
                 option('u', "--update").set(settings.load.update) % "Skip writing flash sectors that already contain identical data" +
                 option('v', "--verify").set(settings.load.verify) % "Verify the data was written correctly" +
-                option('x', "--execute").set(settings.load.execute) % "Attempt to execute the downloaded file as a program after the load"
+                option('x', "--execute").set(settings.load.execute) % "Perform a bootrom reboot to execute the downloaded file as a program after the load - either a flash update boot for binaries in flash, or a RAM image boot for other binaries "
             ).min(0).doc_non_optional(true) % "Post load actions" +
             file_selection % "File to load from" +
             (