scripts: clean up west build/flash/debug help

Just changes to the west help output; no functional changes expected.

Make option descriptions lowercase to match the argparse module's
conventions. When multiple sentences are required, move them to parser
prolog/epilog or argument group description sections.

Clarify some points that have confused multiple people.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
diff --git a/scripts/west_commands/debug.py b/scripts/west_commands/debug.py
index 469dc3d..288363d 100644
--- a/scripts/west_commands/debug.py
+++ b/scripts/west_commands/debug.py
@@ -10,7 +10,7 @@
 
 from west.commands import WestCommand
 
-from run_common import desc_common, add_parser_common, do_run_common
+from run_common import add_parser_common, do_run_common
 
 
 class Debug(WestCommand):
@@ -23,8 +23,7 @@
             dedent('''
             Connect to the board, flash the program, and start a
             debugging session. Use "west attach" instead to attach
-            a debugger without reflashing.\n\n''') +
-            desc_common('debug'),
+            a debugger without reflashing.'''),
             accepts_unknown_args=True)
         self.runner_key = 'debug-runner'  # in runners.yaml
 
@@ -48,8 +47,7 @@
 
             The debug server binds to a known port, and allows client software
             started elsewhere to connect to it and debug the running
-            Zephyr image.\n\n''') +
-            desc_common('debugserver'),
+            Zephyr image.'''),
             accepts_unknown_args=True)
         self.runner_key = 'debug-runner'  # in runners.yaml
 
@@ -67,8 +65,7 @@
             'attach',
             # Keep this in sync with the string in west-commands.yml.
             'interactively debug a board',
-            "Like \"west debug\", but doesn't reflash the program.\n\n" +
-            desc_common('attach'),
+            "Like \"west debug\", but doesn't reflash the program.",
             accepts_unknown_args=True)
         self.runner_key = 'debug-runner'  # in runners.yaml