docs: forgotten entities from the TVM upgrade PR

* `SendOnlyEstimateFee` constant
* `nativeSendMessageReturnForwardFee()` function

docs/src/content/docs/book/message-mode.mdx

@@ -5,17 +5,20 @@ description: "Messages are sent with the mode param of a struct SendParameters.
 
 import { Badge } from '@astrojs/starlight/components';
 
-As it was previously mentioned, messages are sent with the `mode` param of a struct `SendParameters{:tact}`. It's an [`Int{:tact}`][int] value, which is combined from base modes and optional flags, which are also [`Int{:tact}`][int] values.
+As it was previously mentioned, messages sent via [`send(){:tact}`](/ref/core-common#send) function utilize the `mode` param of the `SendParameters{:tact}` structure. The `mode` is an [`Int{:tact}`][int] value, which is combined from base modes and optional flags, which are also [`Int{:tact}`][int] values.
 
 It's possible to use raw [`Int{:tact}`][int] values and manually provide them for the `mode`, but for your convenience there's a set of constants which you may use to construct the compound `mode` with ease. Take a look at the following tables for more information on base modes and optional flags.
 
+Note that there are other [message sending functions](/book/send#message-sending-functions) — they do not use the `SendParameters{:tact}` [Struct](/book/structs-and-messages#structs), but accept the `mode` as one of their parameters.
+
 ## Base modes
 
 Mode value | Constant name                 | Description
 ---------: | :---------------------------- | -----------
 $0$        | <Badge text="Since Tact 1.6 (not released yet)" variant="tip"/> `SendDefaultMode{:tact}` | Ordinary message (default).
 $64$       | `SendRemainingValue{:tact}`   | Carry all the remaining value of the inbound message in addition to the value initially indicated in the new message.
 $128$      | `SendRemainingBalance{:tact}` | Carry all the remaining balance of the current smart contract instead of the value originally indicated in the message.
+$1024$     | <Badge text="Since Tact 1.5" variant="tip"/> `SendOnlyEstimateFee{:tact}` | Doesn't send the message, only estimates the forward fees if the [message sending function](/book/send#message-sending-functions) computes those.
 
 ## Optional flags
 

docs/src/content/docs/ref/core-advanced.mdx

@@ -442,6 +442,24 @@ Attempts to queue more than $255$ messages throw an exception with an [exit code
 
 :::
 
+## nativeSendMessageReturnForwardFee
+
+<Badge text="Available since Tact 1.5" variant="tip" size="medium"/><p/>
+
+```tact
+fun nativeSendMessageReturnForwardFee(cell: Cell, mode: Int): Int;
+```
+
+Similar to [`nativeSendMessage(){:tact}`](#nativesendmessage), but also calculates and returns the [forward fee][forward-fee] in [nanoToncoins][nanotoncoin].
+
+Attempts to queue more than $255$ messages throw an exception with an [exit code 33](/book/exit-codes#33): `Action list is too long`.
+
+:::note
+
+  Prefer using a much more common and user-friendly [`send(){:tact}`](/ref/core-common#send) function unless you have a complex logic that can't be expressed otherwise.
+
+:::
+
 ## nativeReserve
 
 ```tact