\ExplSyntaxOn
\cs_new_protected:Npn \__expkvdoc_patchfootnote:% >>=
  {
    \cs_set:Npx \footnote
      {
        \exp_not:n { \ClassError {expkvdoc} {Footnote~ in~ function} {} }
        \exp_not:o { \footnote }
      }
  }% =<<
\AddToHook{env/function/begin}{\__expkvdoc_patchfootnote:}

\group_begin:
\cs_set_protected:Npn \__expkvdoc_tmp:nN #1#2
  {
    \__kernel_tl_set:Nx #2 { \tl_to_str:n {#1} }
    \tl_replace_all:Nxn #2 { \c_underscore_str } { / }
    \tl_remove_all:Nx   #2 { \c_backslash_str }
    \tl_put_left:Nn #2 { doc/function// }
  }
\cs_if_eq:NNTF \__expkvdoc_tmp:nN \__codedoc_get_hyper_target:nN
  {
    \cs_gset:Npn \__codedoc_get_hyper_target:nN #1#2
      {
        \__kernel_tl_set:Nx #2 { \tl_to_str:n {#1} }
        \tl_replace_all:Nxn #2 { \c_underscore_str } { / }
        \tl_remove_all:Nx   #2 { \c_backslash_str }
        \tl_put_left:Nx #2 { doc/function/ \g__expkvdoc_module_str / }
      }
  }
  {
    \ClassWarning{expkvdoc}
      {
        Didn't~ patch~ l3doc's~ hyper~ targets.~ You~ might~ get~ duplicate~
        labels.
      }
  }
\group_end:

\ExplSyntaxOff