Add clarifying comments.

Commit migrated from https://github.com/dotnet/corefx/commit/5d629d10ab771824ec260f9ae69cab166fdf4cb1

src/libraries/System.Linq/src/System/Linq/Select.cs

@@ -169,6 +169,9 @@ public List<TResult> ToList()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 if (onlyIfCheap)
                 {
                     return -1;
@@ -255,6 +258,9 @@ public List<TResult> ToList()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 if (!onlyIfCheap)
                 {
                     foreach (TSource item in _source)
@@ -388,6 +394,9 @@ public List<TResult> ToList()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 int count = _source.Count;
 
                 if (!onlyIfCheap)
@@ -538,6 +547,9 @@ public List<TResult> ToList()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 int count = _source.Count;
 
                 if (!onlyIfCheap)
@@ -760,6 +772,9 @@ public List<TResult> ToList()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 if (!onlyIfCheap)
                 {
                     foreach (TSource item in _source)
@@ -917,6 +932,9 @@ public List<TResult> ToList()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 int count = Count;
 
                 if (!onlyIfCheap)

src/libraries/System.Linq/src/System/Linq/Where.cs

@@ -434,6 +434,9 @@ public override Iterator<TResult> Clone()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 if (onlyIfCheap)
                 {
                     return -1;
@@ -536,6 +539,9 @@ public override Iterator<TResult> Clone()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 if (onlyIfCheap)
                 {
                     return -1;
@@ -658,6 +664,9 @@ public override void Dispose()
 
             public int GetCount(bool onlyIfCheap)
             {
+                // In case someone uses Count() to force evaluation of
+                // the selector, run it provided `onlyIfCheap` is false.
+
                 if (onlyIfCheap)
                 {
                     return -1;