In this context, multi-page allocations means SQL Server needs more than 8K of memory, and it will allocate multiple 8K pages to hold that data.
This isn't a complete answer, as I admit I don't know the complete answer, but can't fit into a comment.
Short detour - regarding SQLOS memory there is a wealth of information coming from a guy that worked on the internals of SQL Server storage team - Slava Oks. He has lots of articles on MSDN blogs regarding the SQLOS memory manager and they are all worth a read. Now I know we're on to SQL Server 2012, but the info is valuable nonetheless.
Now, regarding your first question, if there's a limit of max memory taken by db page cache before being cleared out and how is it controlled, I'll quote his word on it.
From article SQLOS's memory manager and SQL Server's Buffer Pool:
Buffer Pool commits pages on demand. Depending on internal memory requirements and external memory state, it calculates its target, amount of memory it thinks it should commit before it can get into memory pressure. To keep system out of paging target is constantly recalculated. Target memory can't exceed max memory that represents max server memory settings. Even if you set min server memory equal to max server memory Buffer Pool will only commit its memory on demand. You can observe this behavior by monitoring corresponding profiler event.
From this article's comment I understand that SQL Server uses a LRU algorithm (LRU = least recently used). If you really need to go in depth, SQL Server's LRU algorithm in particular is described in the following research paper: The LRU-K Page Replacement Algorithm For Database Disk Buffering.
Now, for your second question, the log caches are not really exposed so they can't be changed/tuned, as they are a part of the SQLOS internal memory structure. Quoting from an answer from MSDN forum:
Essentially each database has a log buffer/cache for its transaction
log which is a small contiguous memory allocation, that is 60K in size
and used by active transactions, and flushed when a transaction
commits, or when the buffer space fills up. Multiple active
transactions can be interleaved in the log buffer, and if any one
transaction commits, the buffer memory is written to the transaction
log file by the log writer. If the space fills up, the contents get
flushed to disk allowing the buffer/cache memory to be reused. This
is why log IO can range for 512bytes to 64K (its actually 60K), but it
is always sequential so varying write sizes doesn't matter that much.
The format of the log records is different than data pages as
documented in the BOL
(http://msdn.microsoft.com/en-us/library/ms190925.aspx), so the size
of a log record is variable, but the size of the log buffer/cache is
consistent.
Best Answer
In SQL Server 2012 nothing comes from buffer pool anymore, except data pages. This is a change from SQL Server 2008R2. See Memory Manager surface area changes in SQL Server 2012. The buffer pool is just another client of the Memory Manager client for allocations, just like any other memory consumer. So the question is basically unasked in SQL Server 2012.