Replace 'samtools depth' cryptic 'minQLen' with real description

Could anyone understand 'minQLen' in the 'samtools depth' help?

```
$ ./samtools depth

Usage: samtools depth [options] in1.bam [in2.bam [...]]
Options:
   -b <bed>            list of positions or regions
   -f <list>           list of input BAM filenames, one per line [null]
   -l <int>            minQLen
   -q <int>            base quality threshold
   -Q <int>            mapping quality threshold
   -r <chr:from-to>    region
```

Reading the source code, this is mapped to variable ``min_len`` which (if set) is compared to each mapped read's length calculated from the CIGAR string using ``bam_cigar2qlen(b->core.n_cigar, bam_get_cigar(b))``.

I think it makes more sense to label the ``-l`` option as "minimum mapped read length (from CIGAR string)" as per this one-line change.