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/mv.fish

@@ -13,7 +13,7 @@
 # DESCRIPTION
 #   Wraps mv to automatically collapse nested directories of the same name.
 #   When extracting archives results in redundant structures (e.g.,
-#   themes/themes/), calling `mv themes/themes themes` will gracefully
+#   themes/themes/), calling mv themes/themes themes will gracefully
 #   move the inner contents up one level and remove the empty outer shell.
 #
 #   Opinionated component (C1): when disabled via __fish_config_op_aliases,