Escape square brackets when they're not references

[joar]

This fixes warnings like

warning: `[key]` cannot be resolved, ignoring it...
   --> pyo3/src/objectprotocol.rs:155:63
    |
155 |     /// This is equivalent to the Python expression 'del self[key]'.
    |                                                               ^^^ cannot be resolved, ignoring
    |
    = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

when running cargo doc using cargo 1.29.0-nightly (6a7672ef5 2018-08-14) on a crate with pyo3 as a dependency.

[joar]

This could be a bug in cargo though:

• Some of the fixed warnings were caused by square brackets in Python code examples and could be fixed by surrounding Python code examples with backticks
• Some lines already had backticks and required escaping the square brackets inside the backticks in order to silence the warnings.

[joar]

Never mind, the issue is actually that some of the code examples are not surrounded by backticks, but with single quotes. I'll switch those places to using backticks in order to avoid cluttering with backslash in the docstring.

[konstin]

Thank you!