Add docstring to select_b()

Author: ufechner7

docs/src/components.md

@@ -33,6 +33,7 @@ on_timer(rl::RateLimiter)
 ```@docs
 Mixer_2CH
 Mixer_2CH(dt, t_blend)
+select_b(m2::Mixer_2CH, select_b::Bool)
 ```
 
 ## Mixer_3CH

src/components.jl

@@ -280,8 +280,21 @@ function Mixer_2CH(dt, t_blend)
     Mixer_2CH(dt, t_blend, 0, false)
 end
 
+"""
+    select_b(m2::Mixer_2CH, select_b::Bool)
+
+Select input a or b.
+
+## Parameters
+- m2::Mixer_2CH: the two-channel mixer
+- select_b: if true, select channel b, otherwise select channel a
+
+## Returns
+- nothing
+"""
 function select_b(m2::Mixer_2CH, select_b::Bool)
     m2.select_b = select_b
+    nothing
 end
 
 function on_timer(m2::Mixer_2CH)