[HHH-18993] modify the user guide in documentation.

[saleese]

[Please describe here what your change is about]
Regarding the outdated API references, I made the following changes in three adoc files:

1. 1. getDefaultBatchLoadSizingStrategy was deleted but the change has not been reflected in the current version of the user guide at https://github.com/hibernate/hibernate-orm/blob/main/documentation/src/main/asciidoc/userguide/chapters/pc/PersistenceContext.adoc

Before: The default is to use a batch sizing strategy defined by the Dialect.getDefaultBatchLoadSizingStrategy() method.
After: The default is to use a batch sizing strategy defined by the configuration settings in Hibernate.

2. LazyToOneOption was deleted but the change has not been reflected in the current version of the user guide at https://github.com/hibernate/hibernate-orm/blob/main/documentation/src/main/asciidoc/userguide/chapters/pc/BytecodeEnhancement.adoc

Before: As a hopefully temporary legacy hold-over, it is currently required that all lazy singular associations (many-to-one and one-to-one) also include @LazyToOne(LazyToOneOption.NO_PROXY).
After: For existing codebases, it is recommended to remove usages of @LazyToOne, such as @LazyToOne(LazyToOneOption.NO_PROXY), and instead rely on default lazy loading behavior or explicitly configure fetch strategies using @OnetoOne or @manytoone annotations with fetch = FetchType.LAZY where necessary, such as @OneToOne(fetch = FetchType.LAZY).

3. 3. GenerationTime was deleted but the change has not been reflected in the current version of the user guide at https://github.com/hibernate/hibernate-orm/blob/main/documentation/src/main/asciidoc/userguide/chapters/locking/Locking.adoc

Before: If the version number is generated by the database, such as a trigger, use the annotation @org.hibernate.annotations.Generated(GenerationTime.ALWAYS) on the version attribute.
After: If the version number is generated by the database, such as a trigger, implement a custom generator by extending the OnExecutionGenerator interface, and apply it to the version attribute using a custom annotation. Refer to the Hibernate documentation for the latest guidelines on handling database-managed attributes.

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https://hibernate.atlassian.net/browse/HHH-18993

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[yrodiere]

I rebased and applied suggested changes.

I think we're good to merge @saleese @gavinking ?