Remove redundant documentation line

Author: mielvds

docs/reference/flatten.md

@@ -12,9 +12,6 @@
       show_bases: false
       heading_level: 3
 
-Pass an `IdentifierIssuer` instance as the `identifierIssuer` option to control
-blank node identifiers generated during flattening.
-
 ## Example
 
 {{ example('flatten.py', 'json') }}

docs/reference/frame.md

@@ -12,9 +12,6 @@
       show_bases: false
       heading_level: 3
 
-Pass an `IdentifierIssuer` instance as the `identifierIssuer` option to control
-blank node identifiers generated during framing.
-
 ## Example
 
 {{ example('frame.py', 'json') }}

docs/reference/to_rdf.md

@@ -12,9 +12,6 @@
       show_bases: false
       heading_level: 3
 
-Pass an `IdentifierIssuer` instance as the `identifierIssuer` option to control
-blank node identifiers generated during RDF conversion.
-
 ## Example
 
 {{ example('to_rdf.py') }}