Fix remaining legacy section title styles in documentation

Section titles were updated in change I811dd7238 to use asciidoctor's
new style, i.e.

  = Section level 1

  == Section level 2

  === Section level 3

instead of

  Section level 1
  ===============

  Section level 2
  ---------------

  Section level 3
  ~~~~~~~~~~~~~~~

Fix a few more level 3 headers that are using the old style, either
because they were missed by I811dd7238 or because they were added
later and missed during review.

Change-Id: I813bc834272d3cb6c2b759becfda87704a7ac191

Documentation/js-api.txt

@@ -701,8 +701,7 @@ namespace by Gerrit Code Review. All top-level functions can be
 accessed through this name.
 
 [[Gerrit_css]]
-Gerrit.css()
-~~~~~~~~~~~~
+=== Gerrit.css()
 Creates a new unique CSS class and injects it into the document.
 The name of the class is returned and can be used by the plugin.
 See link:#Gerrit_html[`Gerrit.html()`] for an easy way to use
@@ -805,8 +804,7 @@ the current browser window will navigate to the non-Gerrit URL.
 The user can return to Gerrit with the back button.
 
 [[Gerrit_html]]
-Gerrit.html()
-~~~~~~~~~~~~~
+=== Gerrit.html()
 Parses an HTML fragment after performing template replacements.  If
 the HTML has a single root element or node that node is returned,
 otherwise it is wrapped inside a `<div>` and the div is returned.
@@ -900,8 +898,7 @@ screen.onUnload(function() { w.clear() });
 ----
 
 [[Gerrit_injectCss]]
-Gerrit.injectCss()
-~~~~~~~~~~~~~~~~~~
+=== Gerrit.injectCss()
 Injects CSS rules into the document by appending onto the end of the
 existing rule list.  CSS rules are global to the entire application
 and must be manually scoped by each plugin.  For an automatic scoping

Documentation/rest-api-changes.txt

@@ -4740,8 +4740,7 @@ UUID of a draft comment.
 The name of the label.
 
 [[file-id]]
-\{file-id\}
-~~~~~~~~~~~~
+=== \{file-id\}
 The path of the file.
 
 [[revision-id]]

Documentation/rest-api-groups.txt

@@ -1355,8 +1355,7 @@ should be included or deleted.
 |==========================
 
 [[members-input]]
-MembersInput
-~~~~~~~~~~~
+=== MembersInput
 The `MembersInput` entity contains information about accounts that should
 be added as members to a group or that should be deleted from the group.
 

Documentation/rest-api-projects.txt

@@ -2408,8 +2408,7 @@ of parameter names to values.
 |======================================================
 
 [[config-parameter-info]]
-ConfigParameterInfo
-~~~~~~~~~~~~~~~~~~~
+=== ConfigParameterInfo
 The `ConfigParameterInfo` entity describes a project configuration
 parameter.