Clarify tx_insufficient_fee transaction result code

[JakeUrban]

The most likely reason for a 504 Timeout response from Horizon is due to having a fee that is not competitive with the other transactions being considered. However, we also have a tx_insufficient_fee transaction result code, which causes a bit of confusion.

However, it was recently explained to me that tx_insufficient_fee is used when the the fee is below the base fee, which is separate from the 504 Timeout scenario where the fee offered is above the base fee but uncompetitive with other transactions being considered.

[gionapaolini]

Adding on this, tx_insufficient_fee doesn't always relates to fee below the base fee. In fact while doing some load test, it seems some requests fail with this error, even tho fee is set to 1000 (10x base fee).

[arkantos1482]

same happen to me, every now and then some transactions raise this tx_insufficient_fee exception, for 300 trool as fee