MRG: add usage docs for sig import/ingest (#2785)

Fixes #1715

doc/command-line.md

@@ -1704,6 +1704,8 @@ will import the contents of `filename.msh.json` into `imported.sig`.
 
 Note: `import` only creates one output file, with one signature in it.
 
+Note: `ingest` is an alias for `import`.
+
 ### `sourmash signature export` - export signatures to mash.
 
 Export signatures from sourmash format. Currently only supports

src/sourmash/cli/sig/ingest.py

@@ -1,11 +1,23 @@
 """ingest/import a mash or other signature"""
 
+usage="""
+
+   sourmash sig ingest --csv <input filename> [ <more inputs> ] -o <output>
+
+Ingest num sketches from a simple CSV format, or alternatively a JSON
+formatproduced by 'mash info -d'.  The CSV file should contain one
+line per sketch, with the first column containing 'murmur64', the
+second being '42', the third and fourth being the k-mer size and the
+name, and the remaining columns being the hashes.
+
+"""
+
 
 def subparser(subparsers):
     # Dirty hack to simultaneously support new and previous interface
     # If desired, this function can be removed with a major version bump.
     for cmd in ('ingest', 'import'):
-        subparser = subparsers.add_parser(cmd)
+        subparser = subparsers.add_parser(cmd, usage=usage)
         subparser.add_argument('--csv', action='store_true',
                                help='import in Mash CSV format')
         subparser.add_argument('filenames', nargs='+')
@@ -21,4 +33,4 @@ def subparser(subparsers):
 
 def main(args):
     import sourmash
-    return sourmash.sig.__main__.sig_import(args)
+    return sourmash.sig.__main__.ingest(args)

src/sourmash/sig/__main__.py

@@ -15,7 +15,7 @@
            "filter",
            "flatten",
            "downsample",
-           "sig_import",
+           "ingest",
            "export",
            "kmers",
            "fileinfo",
@@ -935,7 +935,7 @@ def downsample(args):
         sourmash_args.report_picklist(args, picklist)
 
 
-def sig_import(args):
+def ingest(args):
     """
     import a signature into sourmash format.
     """