Mention capabilities in extending docs

Author: krassowski

atest/05_Features/Completion.robot

@@ -36,7 +36,6 @@ Filters Completions In Case Sensitive Mode
     Completer Should Suggest    test
     Completer Should Not Suggest    TabError
 
-
 Can Prioritize Kernel Completions
     # note: disabling pre-filtering to get ranking without match scoring
     Configure JupyterLab Plugin    {"kernelCompletionsFirst": true, "kernelResponseTimeout": -1, "preFilterMatches": false}    plugin id=${COMPLETION PLUGIN ID}

atest/05_Features/Diagnostics.robot

@@ -4,7 +4,6 @@ Force Tags        feature:diagnostics
 Test Setup        Setup Notebook    Python    Diagnostic.ipynb
 Test Teardown     Clean Up After Working With File    Diagnostic.ipynb
 Resource          ../Keywords.robot
-
 # note: diagnostics are also tested in 01_Editor and 04_Interface/DiagnosticsPanel.robot
 
 *** Test Cases ***

atest/05_Features/Signature.robot

@@ -62,18 +62,16 @@ Details Should Expand On Click
     Wait Until Keyword Succeeds    20x    0.5s    Page Should Contain Element    ${SIGNATURE_BOX}
     Wait Until Keyword Succeeds    10x    0.5s    Element Should Contain    ${SIGNATURE_BOX}    Short description.
     Page Should Contain Element    ${SIGNATURE_DETAILS}
-    Details Should Be Collapsed     ${SIGNATURE_DETAILS_CSS}
+    Details Should Be Collapsed    ${SIGNATURE_DETAILS_CSS}
     Click Element    ${SIGNATURE_DETAILS}
     Details Should Be Expanded    ${SIGNATURE_DETAILS_CSS}
 
 *** Keywords ***
-
 Details Should Be Expanded
     [Arguments]    ${css_locator}
     ${is_open}    Execute JavaScript    return document.querySelector('${css_locator}').open
     Should Be True    ${is_open} == True
 
-
 Details Should Be Collapsed
     [Arguments]    ${css_locator}
     ${is_open}    Execute JavaScript    return document.querySelector('${css_locator}').open

docs/Extending.ipynb

@@ -34,6 +34,8 @@
     "  `CodeMirrorIntegration` class.\n",
     "- `labIntegration`: an optional object integrating feature with the JupyterLab\n",
     "  interface\n",
+    "- `capabilities`: an optional object defining the [client\n",
+    "  capabilities][clientcapabilities] implemented by your feature,\n",
     "- optional fields for easy integration of some of the common JupyterLab systems,\n",
     "  such as:\n",
     "  - settings system\n",
@@ -46,7 +48,10 @@
     "#### How to override the default implementation of a feature?\n",
     "\n",
     "You can specify a list of extensions to be disabled the the feature manager\n",
-    "passing their plugin identifiers in `supersedes` field of `IFeatureOptions`."
+    "passing their plugin identifiers in `supersedes` field of `IFeatureOptions`.\n",
+    "\n",
+    "[clientCapabilities]:\n",
+    "https://microsoft.github.io/language-server-protocol/specifications/specification-3-17/#clientCapabilities]"
    ]
   },
   {