Explain TX_FLAGS in transfer docs

[pro-wh]

there's a reference to TX_FLAGS in a code snippet, but the page doesn't explain it. it's on a separate page, https://docs.oasis.dev/general/use-your-tokens/setup#storing-base-and-signer-flags-in-an-environment-variable

[tjanez]

Each example transaction page, e.g. https://docs.oasis.dev/general/use-your-tokens/transfer-tokens, has the following info box at the top:

This example assumes you have read and followed the instructions in the Prerequisites and Setup sections.

I guess people don't read that 🙂.