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_settings_get_val.fish

@@ -6,7 +6,7 @@
 #
 # DESCRIPTION
 #   Returns the current value of a named variable in the specified scope by
-#   parsing `set --show` output. Outputs "on", "off", or "DEFAULT" (when
+#   parsing set --show output. Outputs "on", "off", or "DEFAULT" (when
 #   the variable is not set in that scope). Scope "session" maps to "global"
 #   in fish's internal terminology.
 #