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/config-update.fish

@@ -11,7 +11,7 @@
 #   Pulls the latest fish shell configuration from the upstream repository
 #   into ~/.config/fish. Git output is suppressed; status is reported
 #   through colored messages. After a successful pull the function prints a
-#   short summary of changed files; run `exec fish` to reload the shell.
+#   short summary of changed files; run exec fish to reload the shell.
 #
 # ARGUMENTS
 #   -h, --help      Show this help message and exit