Merge pull request #113 from atcoder/patch/issue112

#112: add constraints for maxflow.get_edge

Author: yosupo06

document_en/maxflow.md

@@ -88,6 +88,8 @@ struct mf_graph<Cap>::edge {
 
 **@{keyword.constraints}**
 
+- (1): $0 \leq i \lt m$
+
 **@{keyword.complexity}**
 
 - (1): $O(1)$

document_ja/maxflow.md

@@ -90,6 +90,8 @@ struct mf_graph<Cap>::edge {
 
 **@{keyword.constraints}**
 
+- (1): $0 \leq i \lt m$
+
 **@{keyword.complexity}**
 
 $m$ を追加された辺数として