MSF File Format: Miswording in Free Block Map documentation

Documentation bug here:

https://llvm.org/docs/PDB/MsfFile.html

Documentation says this:

> the first block of each FPM refers to the first 8 intervals of the file (the first 32768 blocks), the second block of each FPM refers to the next 8 blocks, and so on.

But if I'm not mistaken, it should say:

> the first block of each FPM refers to the first 8 intervals of the file (the first 32768 blocks), the second block of each FPM refers to the next 8 **intervals**, and so on.

This was confusing to read so an update to the wording would be appreciated.

--

This part was also hard to understand for me:

> The Free Block Maps are stored as a series of single blocks throughout the file at intervals of BlockSize.

I think the text could use a clarification on the units for the number "BlockSize". I assume the unit is in blocks? So it means something like, "at intervals of `BlockSize` number of blocks"?

Start with the MSF File Format page at https://llvm.org/docs/PDB/MsfFile.html and review the passages describing the Free Block Maps. Clarify the repeated unit wording, including whether BlockSize is measured in blocks, and ensure the revised text accurately describes the intervals.

Written by the indexing model from the issue text.