Make domain bound specs more visually descriptive

[PaulRBerg]

Context

While reviewing #212, it dawned on me that the NatSpec could be made clearer by having the domain bound specs be more visually descriptive.

Comparison Symbols

e.g. instead of:

prb-math/src/ud60x18/Casting.sol

Line 16 in 3681681

/// - x must be less than or equal to `uMAX_SD1x18`.

Write this using the less than or equal symbol:

/// - x ≤ uMAX_SD1x18

Inclusion Symbols

Similarly, this:

prb-math/src/ud2x18/Casting.sol

Line 23 in 3681681

/// @dev There is no overflow check because the domain of UD2x18 is a subset of SD59x18.

Could be turned into this using the inclusion relation symbol:

/// - There is no overflow check because UD2x18 ⊆ SD59x18