Update FasterLog.md

docs/cs/FasterLog.md

@@ -17,7 +17,8 @@ segments corresponding to files on disk. Each segment consists of a fixed number
 and pages sizes are configurable during construction of FasterLog. By default, 
 FasterLog commits at page boundaries. You can also force-commit the log as frequently as you need, e.g., every 
 5ms. The typical use cases of FasterLog are captured in our extremely detailed commented sample [here](https://github.com/microsoft/FASTER/blob/master/cs/playground/FasterLogSample/Program.cs). FasterLog
-works with .NET Standard 2.0, and can be used on a broad range of machines and devices.
+works with .NET Standard 2.0, and can be used on a broad range of machines and devices. We have tested
+it on both Windows and Linux-based machines.
 
 ## Creating the Log
 
@@ -130,9 +131,19 @@ end of iteration, or because we are waiting for a page read or commit to complet
 For tailing iteration, you simply specify `long.MaxValue` as the end address for the scan. You are guaranteed
 to see only committed entries during a tailing iteration.
 
-You can persist iterators as part of commit by simply naming them during their creation. On recovery, if an
-iterator with the specified name exists, it will be initialized with the corresponding current iterator 
-pointer.
+An iterator is associated with a `CompletedUntilAddress` which indicates until what address the iteration has been
+completed. Users update the `CompletedUntilAddress` as follows:
+
+```cs
+iter.CompleteUntil(long address);
+```
+
+The specified address needs to be a valid log address, similar to the `TruncateUntil` call on the log, described 
+earlier.
+
+You can persist iterators (or more precisely, their `CompletedUntilAddress`) as part of a commit by simply naming 
+them during their creation. On recovery, if an iterator with the specified name exists, it will be initialized with
+its last-committed `CompletedUntilAddress` as the current iterator pointer.
 
 ```cs
 using (var iter = log.Scan(0, long.MaxValue, name: "foo"))
@@ -149,10 +160,18 @@ using (var iter = log.Scan(0, long.MaxValue, name: "foo", recover: false))
 
 FasterLog support log head truncation, until any prefix of the log. Truncation updates the log begin address and
 deletes truncated parts of the log from disk, if applicable. A truncation is persisted via commit, similar 
-to tail address commit. Here is an example, where we truncate the log to limit it to the last 100GB:
+to tail address commit.
+
+There are two variants: `TruncateUntilPageStart` truncates until the start of the page corresponding to the 
+specified address. This is safe to invoke with any address, as the page start is always a valid pointer to the
+log. The second variant, called `TruncateUntil`, truncates exactly until the specified address. Here, the user 
+needs to be careful and provide a valid address that points to a log entry. For example, any address returned by 
+an iterator is a valid location that one could truncate until.
+
+Here is an example, where we truncate the log to limit it to the last 100GB:
 
 ```cs
-  log.TruncateUntil(log.CommittedUntilAddress - 100_000_000_000L);
+  log.TruncateUntilPageStart(log.CommittedUntilAddress - 100_000_000_000L);
 ```
 
 ### Random Read