docs: networking: convert mac80211-injection.txt to ReST

- add SPDX header;
- adjust title markup;
- mark code blocks and literals as such;
- mark tables as such;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index 82846ac..9849c14 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -2144,7 +2144,7 @@ static bool ieee80211_parse_tx_radiotap(struct ieee80211_local *local,
 
 		/*
 		 * Please update the file
-		 * Documentation/networking/mac80211-injection.txt
+		 * Documentation/networking/mac80211-injection.rst
 		 * when parsing new fields here.
 		 */