Collection of typo fixes.

Use "a" and "an" correctly, mostly in comments. Two error messages were
also fixed (they were just elogs, so no translation work required). Two
function comments in pg_proc.h were also fixed. Etsuro Fujita reported one
of these, but I found a lot more with grep.

Also fix a few other typos spotted while grepping for the a/an typos.
For example, "consists out of ..." -> "consists of ...". Plus a "though"/
"through" mixup reported by Euler Taveira.

Many of these typos were in old code, which would be nice to backpatch to
make future backpatching easier. But much of the code was new, and I didn't
feel like crafting separate patches for each branch. So no backpatching.

src/backend/utils/misc/sampling.c

@@ -78,7 +78,7 @@ BlockSampler_Next(BlockSampler bs)
 	 * Knuth says to skip the current block with probability 1 - k/K.
 	 * If we are to skip, we should advance t (hence decrease K), and
 	 * repeat the same probabilistic test for the next block.  The naive
-	 * implementation thus requires an sampler_random_fract() call for each
+	 * implementation thus requires a sampler_random_fract() call for each
 	 * block number.  But we can reduce this to one sampler_random_fract()
 	 * call per selected block, by noting that each time the while-test
 	 * succeeds, we can reinterpret V as a uniform random number in the range