typos

Most of this is new in v15 or doesn't affect user-facing docs so doesn't need
to be backpatched.

Feel free to ignore this for now and revisit in April...

@Michael: I'm not sure what this is trying to say.
1e9475694b0ae2cf1204d01d2ef6ad86f3c7cac8
+ First, scan the directory where the WAL segment files are written and
+ find the newest completed segment file, using as starting point the
+ beginning of the next WAL segment file. This is calculated independently
+ on the compression method used to compress each segment.

I suppose it should say independently *of* the compression method, but then I
still don't know what it means. I checked FindStreamingStart().
It that doesn't look like it's "calculated independently" - actually, it takes
the compression method into account and explicitly handles each compression
method.

Is there any reason the user-facing docs need to say anything about this at
all?