docs(functions): drop backticks from doc-headers

CONTRIBUTING states doc-headers are written as plain text -- the header
is read as-is by config-help, by funcsave, and by anyone opening the
file, and docs/codespans.py adds the site's inline code spans at render
time. 22 files had drifted from that, carrying 41 hand-written spans
that reached config-help and the man page as literal backtick
characters inside an otherwise verbatim block.

The one span whose content ended in a space is requoted rather than
dropped, so "read> " keeps reading as a prompt string.

functions/smart_exit.fish

@@ -31,7 +31,7 @@
 #
 # NOTES
 #   The exit builtin is wired to smart_exit for interactive sessions. Typing
-#   `exit` or Ctrl+D behaves identically to calling smart_exit directly.
+#   exit or Ctrl+D behaves identically to calling smart_exit directly.
 function smart_exit --description 'Capture colorized scrollback before exiting, with pruning and safe overrides'
     # Opinionated guard (C3): exit plainly when overrides are disabled.
     # This composes with Task #4's __fish_config_enable_logging, which will