Add 'CamelCase' to doc_valid_idents

[phansch]

Fixes #2395

[phansch]

huh, AppVeyor fails with:

   Compiling failure v0.1.3
error[E0034]: multiple applicable items in scope
   --> C:\Users\appveyor\.cargo\registry\src\github.com-1ecc6299db9ec823\failure-0.1.3\src\backtrace\mod.rs:132:20
    |
132 |                 bt.fmt(f)
    |                    ^^^ multiple `fmt` found
    |
    = note: candidate #1 is defined in an impl of the trait `std::fmt::Debug` for the type `backtrace::backtrace::Backtrace`
    = note: candidate #2 is defined in an impl of the trait `std::fmt::Display` for the type `backtrace::backtrace::Backtrace`

[phansch]

Seems like that will be fixed by rust-lang-deprecated/failure#279

[phansch]

@bors r+ rollup

[bors]

📌 Commit ee2abc3 has been approved by phansch

[bors]

⌛ Testing commit ee2abc3 with merge f26c39b...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: phansch
Pushing f26c39b to master...