Improve the API section of the readme

[ehmicky]

This improves the API section:

• the description of each property of the return/resolved value (or thrown/rejected error) was missing.
• the return value of execa.sync() was wrong (it is not the same as the return value of spawnSync() although it has some common properties.
• the extra methods and properties of the child_process instance returned by the main function are now separate.
• the description of the main function has been improved.

To be more efficient, I will update the index.d.ts file after a first code review.

[ehmicky]

Fixed! I've also fixed the index.d.ts.

[sindresorhus]

Looks good :)