Improve snippet information in cheatsheet

[pokey]

• [ ] Add action entry for snip <snippet> <destination>
• [ ] Add action entry for snip <snippet> <phrase>
• [ ] Add action entry for <wrapper_snippet> wrap <target>
• [ ] Add box with list of wrapper snippets
• [ ] Add box with list of insertion snippets
• [ ] Add box with list of single-phrase insertion snippets (merge with insertion snippets box?)