docs: mention that you can merge multiple GMT files for fgsea (#159)

This change allows for multiple gene set files to be specified.

There are two things to especially consider in reviewing this:

1. Do we even want to enable this, or do we want to expect people to
merge their `.gmt` files manually, to have just one comprehensive run of
fgsea? This might be the favorable, as this would ensure multiple
testing control across all specified gene sets at once. If we decide to
do this, I would create a separate pull request to properly explain this
in the `config.yaml` (and possiblye `README.md`) files.
2. Do we want to make this another breaking change, or can we simply
tack this onto yesterday's `3.0.0` release with a minor version bump?

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Improved comments in configuration files to clarify requirements and
best practices for providing gene set files when using the enrichment
tool, including instructions on merging gene sets and citing sources. No
changes to actual configuration options.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: dlaehnemann

.test/config/config.yaml

@@ -91,6 +91,11 @@ enrichment:
     fdr_genes: 0.05
     fdr_go_terms: 0.05
   fgsea:
+    # If activated, you need to provide a GMT file with gene sets of interest.
+    # A GMT file can contain multiple gene sets. So if you want to test for
+    # enrichment in multiple sets of gene sets, please merge your gene sets
+    # into one GMT file and provide this here. Don't forget to document your
+    # gene set sources and to cite them upon publication.
     gene_sets_file: "ngs-test-data/ref/dummy.gmt"
     # tool is only run if set to `true`
     activate: true

.test/three_prime/config/config.yaml

@@ -91,6 +91,11 @@ enrichment:
     fdr_genes: 0.05
     fdr_go_terms: 0.05
   fgsea:
+    # If activated, you need to provide a GMT file with gene sets of interest.
+    # A GMT file can contain multiple gene sets. So if you want to test for
+    # enrichment in multiple sets of gene sets, please merge your gene sets
+    # into one GMT file and provide this here. Don't forget to document your
+    # gene set sources and to cite them upon publication.
     gene_sets_file: "config/gene_sets.gmt"
     # tool is only run if set to `true`
     activate: true

config/config.yaml

@@ -93,7 +93,11 @@ enrichment:
   fgsea:
     # tool is only run if set to `true`
     activate: false
-    # if activated, you need to provide a GMT file with gene sets of interest
+    # If activated, you need to provide a GMT file with gene sets of interest.
+    # A GMT file can contain multiple gene sets. So if you want to test for
+    # enrichment in multiple sets of gene sets, please merge your gene sets
+    # into one GMT file and provide this here. Don't forget to document your
+    # gene set sources and to cite them upon publication.
     gene_sets_file: "resources/gene_sets/dummy.gmt"
     fdr_gene_set: 0.05
     # minimum achievable p-value  (equivalent to the reciprocal of the number of permutations, see fgsea docs)