Document what is the value of maxrss field when not available?

[stepancheg]

Currently it says:

    /// Maximum resident set size
    ///
    /// Expressed in bytes when available.
    pub maxrss: u64,

What is the value when it is not available? Should it be None to mark the value is not available?

Also, it is not very clear it is in bytes, not in megabytes. Could be:

    /// Maximum resident set size, in bytes.
    ///
    /// `None` when not available.
    pub maxrss: Option<u64>,

[lu-zero]

It is 0, I'd rather avoid Option in those cases. Ideally all the platforms supported provide the maxrss.

I'll update the documentation soon, thank you for noticing.