Fix documentation for log file growth.

This fixes #546 reported on GitHub.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194549692

doc/impl.md

@@ -65,12 +65,14 @@ Other files used for miscellaneous purposes may also be present (LOCK, *.dbtmp).
 ## Level 0
 
 When the log file grows above a certain size (4MB by default):
-Create a brand new memtable and log file and direct future updates here
+Create a brand new memtable and log file and direct future updates here.
+
 In the background:
-Write the contents of the previous memtable to an sstable
-Discard the memtable
-Delete the old log file and the old memtable
-Add the new sstable to the young (level-0) level.
+
+1. Write the contents of the previous memtable to an sstable.
+2. Discard the memtable.
+3. Delete the old log file and the old memtable.
+4. Add the new sstable to the young (level-0) level.
 
 ## Compactions