Added docstring for endogenous retirement constraint

src/multi_stage/endogenous_retirement.jl

@@ -10,6 +10,19 @@ inputs:
   * stage\_lens – An Int array representing the length $L$ of each model stage.
 
 returns: An Int representing the model stage in before which the resource must retire due to endogenous lifetime retirements.
+
+```math
+\begin{aligned}
+& RETCAP_{y,p} \geq \sum^p_{t=1} MINRET_{y,t} + \sum^r_{t=1}CAP_{y,t} - \sum^p_{t=1}RETCAP_{y,p}
+\end{aligned}
+```
+where $r \in \{1, ..., (p-1)\}$ is defined as the last stage such that if we built $y$ at the end of stage $r$, it would reach its end of life before the end of stage $p$.
+In other words, it is the largest index $r \in \{1, ..., (p-1)\}$ such that:
+```math
+\begin{aligned}
+\sum^p_{t=r+1}StageLength[t] \leq LifeTime[y]
+\end{aligned}
+```
 """
 function get_retirement_stage(cur_stage::Int, lifetime::Int, stage_lens::Array{Int, 1})
 	years_from_start = sum(stage_lens[1:cur_stage]) # Years from start from the END of the current stage