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/_fzf_preview_media.fish

@@ -9,7 +9,7 @@
 #   by a desktop file manager (Dolphin, Nautilus, GNOME Videos, ...) in the
 #   freedesktop thumbnail cache and renders it via _fzf_preview_image if
 #   found. Otherwise falls back to ffprobe-formatted metadata (duration,
-#   codec, resolution, tags) when ffprobe is installed, or plain `file`
+#   codec, resolution, tags) when ffprobe is installed, or plain file
 #   output as a last resort. Neither the thumbnail cache lookup nor ffprobe
 #   are tracked in fish-deps: both are best-effort, matching how the
 #   image-preview tool chain (kitten/chafa/viu/timg) is already handled.