doc: add r and R string interpolation switch examples (#20473)

doc/docs.md

@@ -704,6 +704,13 @@ println('[${10.0000:.2}]') // remove insignificant 0s at the end => [10]
 println('[${10.0000:.2f}]') // do show the 0s at the end, even though they do not change the number => [10.00]
 ```
 
+V also has `r` and `R` switches, which will repeat the string the specified amount of times.
+
+```v
+println('[${'abc':3r}]') // [abcabcabc]
+println('[${'abc':3R}]') // [ABCABCABC]
+```
+
 #### String operators
 
 ```v
@@ -7299,4 +7306,4 @@ Assignment Operators
 +=   -=   *=   /=   %=
 &=   |=   ^=
 >>=  <<=  >>>=
-```
+```