Merge bitcoin/bitcoin#29813: doc: i2p: improve -i2pacceptincoming mention

2179e2c doc: i2p: improve `-i2pacceptincoming` mention (brunoerg)

Pull request description:

  In i2p documentation, it says that "the first time Bitcoin Core connects to the I2P router,
  it automatically generates a persistent I2P address and its corresponding private key by
  default _**or if `-i2pacceptincoming=1` is set**_". This is weird, because `-i2pacceptincoming=1`
  by itself does not have any effect. Moreover, `-i2pacceptincoming` is 1 by default anyway.

ACKs for top commit:
  laanwj:
    This documentation change is correct and makes the documentation slightly shorter, thus easier to read. ACK 2179e2c
  davidgumberg:
    ACK bitcoin/bitcoin@2179e2c
  achow101:
    ACK 2179e2c
  byaye:
    ACK 2179e2c

Tree-SHA512: 18a6a627343fb0aa824029d99df8a232153ba288ce94ec8c5da25693885237381fba505ea1e71c756b2a611243a302d319ca7ae03b526020cd6588710fc2ac17

Author: achow101

doc/i2p.md

@@ -79,8 +79,8 @@ one of the networks has issues.
 ## Persistent vs transient I2P addresses
 
 The first time Bitcoin Core connects to the I2P router, it automatically
-generates a persistent I2P address and its corresponding private key by default
-or if `-i2pacceptincoming=1` is set.  The private key is saved in a file named
+generates a persistent I2P address and its corresponding private key by default,
+unless `-i2pacceptincoming=0` is set.  The private key is saved in a file named
 `i2p_private_key` in the Bitcoin Core data directory.  The persistent I2P
 address is used for making outbound connections and accepting inbound
 connections.