docs: correct %d and document %f in the formatters table

[JiuYue0820]

Problem

The formatters table documents %d as "Number (both integer and float)". That is inaccurate and matches issue #913, where a float passed to %d is truncated: in Node.js the log call goes through util.formatWithOptions (see src/node.js log()), whose %d is printf-style integer formatting. The table also omits %f, which util.format supports for floats.

Solution

• Describe %d as integer formatting (floats truncated toward zero).
• Add a %f row for floating-point numbers.

Node example confirming current behavior:

const createDebug = require('./src/node')('t');
createDebug.log = (...a) => console.log(require('util').formatWithOptions({}, ...a));
createDebug('int %d float %f', 3.7, 3.7); // int 3 float 3.7

Verification

• %d / %f behavior verified against util.format() on current Node.js.
• No code change; documentation only.