Auto merge of #28723 - carols10cents:0-or-O-or-o, r=steveklabnik

Especially when documenting the use of `0`, since zero looks very
similar to `O` in fonts not meant for displaying code.

Other literal characters, traits, etc should also use code formatting.
This change makes this documentation more internally consistent.

Before this change, circled is the character I was using this documentation to find out about and that confused me when it wasn't immediately clear what character it was:

<img width="1013" alt="screen shot 2015-09-28 at 10 13 31 pm" src="https://cloud.githubusercontent.com/assets/193874/10154708/c70815fe-6638-11e5-9acc-57c73a524203.png">

After this change:

<img width="981" alt="screen shot 2015-09-28 at 11 26 35 pm" src="https://cloud.githubusercontent.com/assets/193874/10154710/ce73eeb2-6638-11e5-98f7-902f58679316.png">

I ran `make check-docs` and didn't break anything ⭐

Author: bors

src/libcollections/fmt.rs

@@ -218,7 +218,7 @@
 //! }
 //! ```
 //!
-//! ### fmt::Display vs fmt::Debug
+//! ### `fmt::Display` vs `fmt::Debug`
 //!
 //! These two formatting traits have distinct purposes:
 //!
@@ -358,24 +358,24 @@
 //! to ensure padding is applied is to format your input, then use this
 //! resulting string to pad your output.
 //!
-//! ## Sign/#/0
+//! ## Sign/`#`/`0`
 //!
 //! These can all be interpreted as flags for a particular formatter.
 //!
-//! * '+' - This is intended for numeric types and indicates that the sign
+//! * `+` - This is intended for numeric types and indicates that the sign
 //!         should always be printed. Positive signs are never printed by
 //!         default, and the negative sign is only printed by default for the
-//!         `Signed` trait. This flag indicates that the correct sign (+ or -)
+//!         `Signed` trait. This flag indicates that the correct sign (`+` or `-`)
 //!         should always be printed.
-//! * '-' - Currently not used
-//! * '#' - This flag is indicates that the "alternate" form of printing should
+//! * `-` - Currently not used
+//! * `#` - This flag is indicates that the "alternate" form of printing should
 //!         be used. The alternate forms are:
 //!     * `#?` - pretty-print the `Debug` formatting
-//!     * `#x` - precedes the argument with a "0x"
-//!     * `#X` - precedes the argument with a "0x"
-//!     * `#b` - precedes the argument with a "0b"
-//!     * `#o` - precedes the argument with a "0o"
-//! * '0' - This is used to indicate for integer formats that the padding should
+//!     * `#x` - precedes the argument with a `0x`
+//!     * `#X` - precedes the argument with a `0x`
+//!     * `#b` - precedes the argument with a `0b`
+//!     * `#o` - precedes the argument with a `0o`
+//! * `0` - This is used to indicate for integer formats that the padding should
 //!         both be done with a `0` character as well as be sign-aware. A format
 //!         like `{:08}` would yield `00000001` for the integer `1`, while the
 //!         same format would yield `-0000001` for the integer `-1`. Notice that
@@ -390,8 +390,8 @@
 //!
 //! The default fill/alignment for non-numerics is a space and left-aligned. The
 //! defaults for numeric formatters is also a space but with right-alignment. If
-//! the '0' flag is specified for numerics, then the implicit fill character is
-//! '0'.
+//! the `0` flag is specified for numerics, then the implicit fill character is
+//! `0`.
 //!
 //! The value for the width can also be provided as a `usize` in the list of
 //! parameters by using the `2$` syntax indicating that the second argument is a