Tweak TSV library name docs

Author: jfy133

docs/usage.md

@@ -350,9 +350,9 @@ Note the following important points and limitations for setting up:
 * The TSV must use actual tabs (not spaces) between cells.
 * The input FASTQ filenames are discarded after FastQC, all other downstream results files are based on `Sample_Name`, `Library_ID` and `Lane` columns for filenames.
 * *File* names must be unique regardless of file path, due to risk of over-writing (see: [https://github.com/nextflow-io/nextflow/issues/470](https://github.com/nextflow-io/nextflow/issues/470)).
-  * At different stages of the merging process, (as above) nf-core/eager will use as output filenames the information from the `Sample_Name`, `Library_ID` and `Lane` column columns for filenames.
-  * In other words, your .tsv file must not have rows with `Library1` and `Library1` for both `SampleA` and `SampleB`. While nf-core/eager would not try to _merge_ these, in some stages of the pipeline output files names would be the same, and would overwrite the other if the files are output to the same `results/` subdirectory.
-  * If it is 'too late' and you already have duplicate file names, a workaround is to concatenate the FASTQ files together and supply this to a nf-core/eager run. The only downside is that you will not get independent FASTQC results for each file.
+  * At different stages of the merging process, (as above) nf-core/eager will use as output filenames the information from the `Sample_Name`, `Library_ID` and/or `Lane` columns for filenames.
+  * Library_IDs must be unique (other than if they are spread across multiple lanes). For example, your .tsv file must not have rows with both the strings in the Library_ID column as `Library1` and `Library1`, for **both** `SampleA` and `SampleB` in the Sample_ID column, otherwise the two `Library1.fq.gz` files may result in a filename collision.
+  * If it is 'too late' and you already have duplicated FASTQ file names before starting a run, a workaround is to concatenate the FASTQ files together and supply this to a nf-core/eager run. The only downside is that you will not get independent FASTQC results for each file.
 * Lane IDs must be unique for each sequencing of each library.
   * If you have a library sequenced e.g. on Lane 8 of two HiSeq runs, you can give a fake lane ID (e.g. 20) for one of the FASTQs, and the libraries will still be processed correctly.
   * This also applies to the SeqType column, i.e. with the example above, if one run is PE and one run is SE, you need to give fake lane IDs to one of the runs as well.