Fix some documentation build warnings.

This commit have following changes to fix warnings when building
with Sphinx.

* fixes broken cross-refenrece target
* changes to the appropriate lexer alias for plpgsql
* comments out html_static_path because it doesn't have any static data

Reported-by: Julien Rouhaud
Reviewed-by: Michael Paquier
Author: Masahiro Ikeda
Backpatch-through: 14

docs/conf.py

@@ -24,7 +24,7 @@
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-html-output
 
 html_theme = 'sphinx_rtd_theme'
-html_static_path = ['_static']
+#html_static_path = ['_static']
 
 # Internationalization
 # https://www.sphinx-doc.org/en/master/usage/advanced/intl.html

docs/functional_limitations.md

@@ -1,3 +1,5 @@
+(functional-limitations)=
+
 # Functional limitations
 
 ## Influence of planner GUC parameters

docs/hint_details.md

@@ -44,7 +44,7 @@ underscores, commas and parentheses.  In the following example,
     preceding comments are not sent as a part of this query.
 
 
-```sql
+```plpgsql
 =# CREATE FUNCTION hints_func(integer) RETURNS integer AS $$
    DECLARE
      id  integer;

docs/hint_table.md

@@ -144,7 +144,7 @@ the planner method configuration parameters.  When multiple hints change the
 same GUC, the last hint takes effect.
 [GUC parameters for `pg_hint_plan`](#guc-parameters-for-pg_hint_plan) are also
 settable by this hint but it may not work as expected.
-See [Restrictions](#restrictions) for details.
+See [Functional limitations](#functional-limitations) for details.
 
 ```sql
 =# /*+ Set(random_page_cost 2.0) */