Documentation for SET IFEQ (#189)

This PR adds the documentation to support
valkey-io/valkey#1324

---------

Signed-off-by: Sarthak Aggarwal <[email protected]>
Signed-off-by: Viktor Söderqvist <[email protected]>
Co-authored-by: Viktor Söderqvist <[email protected]>

Author: sarthakaggarwal97

commands/set.md

@@ -12,23 +12,40 @@ The `SET` command supports a set of options that modify its behavior:
 * `PXAT` *timestamp-milliseconds* -- Set the specified Unix time at which the key will expire, in milliseconds (a positive integer).
 * `NX` -- Only set the key if it does not already exist.
 * `XX` -- Only set the key if it already exists.
+* `IFEQ` *comparison-value* -- Set the key if the comparison value matches the existing value. An error is returned and `SET` aborted if the value stored at key is not a string.
 * `KEEPTTL` -- Retain the time to live associated with the key.
-* `!GET` -- Return the old string stored at key, or nil if key did not exist. An error is returned and `SET` aborted if the value stored at key is not a string.
+* `GET` -- Return the old string stored at key, or nil if key did not exist. An error is returned and `SET` aborted if the value stored at key is not a string.
 
 Note: Since the `SET` command options can replace `SETNX`, `SETEX`, `PSETEX`, `GETSET`, it is possible that in future versions of Valkey these commands will be deprecated and finally removed.
 
 ## Examples
 
+Basic usage
 ```
 127.0.0.1:6379> SET mykey "Hello"
 OK
 127.0.0.1:6379> GET mykey
 "Hello"
-127.0.0.1:6379> 
+```
+
+Set a value and an expiry time.
+```
 127.0.0.1:6379> SET anotherkey "will expire in a minute" EX 60
 OK
 ```
 
+Conditionally set a value.
+```
+127.0.0.1:6379> SET foo "Initial Value"
+OK
+127.0.0.1:6379> GET foo
+"Initial Value"
+127.0.0.1:6379> SET foo "New Value" IFEQ "Initial Value"
+OK
+127.0.0.1:6379> GET foo
+"New Value"
+```
+
 ## Patterns
 
 **Note:** The following pattern is discouraged in favor of [the Redlock algorithm](../topics/distlock.md) which is only a bit more complex to implement, but offers better guarantees and is fault tolerant.