Enforce that all flags must have a non-empty `help` string to prevent undocumented flags and reduce technical debt.

PiperOrigin-RevId: 869667593

absl/flags/_flag.py

diff --git a/absl/flags/_flag.py b/absl/flags/_flag.py
index 16d26ff..47c0115 100644
--- a/absl/flags/_flag.py
+++ b/absl/flags/_flag.py
@@ -106,7 +106,7 @@
     self.name = name
 
     if not help_string:
-      help_string = '(no help available)'
+      raise ValueError(f'Flag --{name} MUST have a non-empty help string. Undocumented flags are technical debt.')
 
     self.help = help_string
     self.short_name = short_name