Is there a way to hide a macro pattern from docs?

One option is to have a dummy macro with only the public arms, and use attributes to choose which one rustdoc sees:

/// Do a thing
///
/// ```
/// # extern crate my_crate;
/// # use my_crate::mc;
/// assert_eq!(mc!(58), 100);
/// ```
#[cfg(doc)]
#[macro_export]
macro_rules! mc {
    ($arg:expr) => { ... };
}

#[cfg(not(doc))]
#[macro_export]
macro_rules! mc {
    (@impl, $arg:expr) => { 42 + $arg };
    ($arg:expr) => { mc!(@impl, $arg) };
}

Note: this will only work if you don't use the macro internally.

I guess this is the optimum solution:

/// Not meant to be called directly
#[doc(hidden)]
#[macro_export]
macro_rules! hidden {
    ( $hidden_rule1:expr ) => { ... };
    ( $hidden_rule2:expr ) => { ... };
    ...
}

#[macro_export]
macro_rules! public {
    ( $public:expr ) => ( hidden!($public) );
}

This uses a separate hidden macro (which will probably need to be public) but which is not part of the documentation. All the rules that should be hidden will be hidden and the public one will be visible in the public macro which is part of the documentation.