scripts/decode_stacktrace.sh: clarify command line

The syntax as expressed by usage() is not entirely correct: "<modules
path>" cannot be passed without "<base path>|auto".  Additionally human
reading of this syntax can be subject to misunderstanding due the mixture
of '|' and '[]'.

Improve readability in various ways:
 * rewrite using two lines for the two allowed usages
 * add square brackets around "<vmlinux>" as it is optional when using
   debuginfod-find
 * move "<modules path>" to inside the square brackets of the 2nd
   positional parameter
 * use underscores instead of spaces in <...> strings

Link: https://lkml.kernel.org/r/20240823-decode_stacktrace-find_module-improvements-v2-2-d7a57d35558b@bootlin.com
Signed-off-by: Luca Ceresoli <[email protected]>
Reviewed-by: Stephen Boyd <[email protected]>
Cc: Alexis Lothoré (eBPF Foundation) <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Thomas Petazzoni <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

scripts/decode_stacktrace.sh

@@ -5,7 +5,8 @@
 
 usage() {
 	echo "Usage:"
-	echo "	$0 -r <release> | <vmlinux> [<base path>|auto] [<modules path>]"
+	echo "	$0 -r <release>"
+	echo "	$0 [<vmlinux> [<base_path>|auto [<modules_path>]]]"
 }
 
 # Try to find a Rust demangler