nushell: fix 0.114.0 nushell str downcase deprecated error

We switch to `str lowercase` and add a note for the Nushell<0.114.0 user
that should display at runtime.

Close #4857

shell/completion.nu

@@ -116,7 +116,8 @@ def __fzf_list_hosts [] {
     (
       # Process ssh config files
       $ssh_configs | append $ssh_configs_d | append $ssh_config_global
-                   | where {|it| ($it | str downcase | str starts-with 'host') or ($it | str downcase | str starts-with 'hostname') }
+                     # NOTE: str lowercase is new from Nushell 0.114.0. Please upgrade to Nushell >=0.114.0 if you are seeing `extra positional argument` error regarding `str lowercase` in this block.
+                   | where {|it| ($it | str lowercase | str starts-with 'host') or ($it | str lowercase | str starts-with 'hostname') }
                    | parse --regex '^\s*host(?:name)?\s+(?<hosts>.+)' # Extract hosts after keyword
                    | default { hosts: null }                          # Handle lines that don't match regex
                    | get hosts